Issue:
Bug: [FEATURE]: [Fiserv] Currency Unit Conversion



### :memo: Feature Description

- Each currency can be described in terms of base or minor units. 
- For instance, when dealing with USD currency, the Cent is considered as the minor unit, while the Dollar is considered the base unit. 
- In Hyperswitch, the amount value is expected to be always provided in minor units. 
- For example: In USD, If the amount is 1 in base unit (Dollar), then it will be equivalent to 100 in minor units (Cent)
- Some of the connectors integrated require the amount to be converted to a desirable unit before being passed to them. 
- We have functions `to_currency_base_unit` and `to_currency_lower_unit` in place to convert the minor unit amount to its decimal equivalent and vice versa. These conversions are handled explicitly based on the connector.


### :hammer: Possible Implementation

- ConnectorCommon trait have been implemented for the connector. 
- This trait contains `get_currency_unit` method. This method needs to be implemented. 
- It will define what type of conversion needs to be done during `connector_router_data` creation. Concurrently handle the creation of `connector_router_data` to be passed for the connector request body creation.
- You can check this PR for further reference https://github.com/juspay/hyperswitch/pull/2196

:bookmark: Note: All the changes needed should be contained within `hyperswitch/crates/router/src/connector/`

### :package: Have you spent some time checking if this feature request has been raised before?

- [X] I checked and didn't find a similar issue

### :package: Have you read the Contributing Guidelines?

- [X] I have read the [Contributing Guidelines](https://github.com/juspay/hyperswitch/blob/main/docs/CONTRIBUTING.md)

### :sparkles: Are you willing to submit a PR?


Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [x] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
- Addressing Issue: #2225 
- Modified two files in `hyperswitch/crates/router/src/connector/`
  - `fiserv.rs`
    - Implement `get_currency_unit` function
    - Modify `ConnectorIntegration` implementations for `Fiserv`
  - `fiserv/transformers.rs`
    - Implement `FiservRouterData<T>` structure and functionality
    - Modify implementation for `FiservPaymentsRequest` and `FiservRefundRequest`


### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!--
Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
<!--
Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

If you don't have an issue, we'd recommend starting with one first so the PR
can focus on the implementation (unless it is an obvious bug or documentation fix
that will have little conversation).
-->


## How did you test it?
<!--
Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
-->
We need to create a payment using Fiserv and test the currency unit from connector response and connector dashboard.

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [ ] I addressed lints thrown by `cargo clippy`
- [ ] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible
- [ ] I added a [CHANGELOG](/CHANGELOG.md) entry if applicable


Testing:

We need to create a payment using Fiserv and test the currency unit from connector response and connector dashboard.

Patch:
diff --git a/crates/router/src/connector/fiserv.rs b/crates/router/src/connector/fiserv.rs
index 70f58ffe6eb..35d40f1a3fb 100644
--- a/crates/router/src/connector/fiserv.rs
+++ b/crates/router/src/connector/fiserv.rs
@@ -104,6 +104,10 @@ impl ConnectorCommon for Fiserv {
         "fiserv"
     }
 
+    fn get_currency_unit(&self) -> api::CurrencyUnit {
+        api::CurrencyUnit::Base
+    }
+
     fn common_get_content_type(&self) -> &'static str {
         "application/json"
     }
@@ -400,7 +404,13 @@ impl ConnectorIntegration<api::Capture, types::PaymentsCaptureData, types::Payme
         &self,
         req: &types::PaymentsCaptureRouterData,
     ) -> CustomResult<Option<types::RequestBody>, errors::ConnectorError> {
-        let connector_request = fiserv::FiservCaptureRequest::try_from(req)?;
+        let router_obj = fiserv::FiservRouterData::try_from((
+            &self.get_currency_unit(),
+            req.request.currency,
+            req.request.amount_to_capture,
+            req,
+        ))?;
+        let connector_request = fiserv::FiservCaptureRequest::try_from(&router_obj)?;
         let fiserv_payments_capture_request = types::RequestBody::log_and_get_request_body(
             &connector_request,
             utils::Encode::<fiserv::FiservCaptureRequest>::encode_to_string_of_json,
@@ -505,7 +515,13 @@ impl ConnectorIntegration<api::Authorize, types::PaymentsAuthorizeData, types::P
         &self,
         req: &types::PaymentsAuthorizeRouterData,
     ) -> CustomResult<Option<types::RequestBody>, errors::ConnectorError> {
-        let connector_request = fiserv::FiservPaymentsRequest::try_from(req)?;
+        let router_obj = fiserv::FiservRouterData::try_from((
+            &self.get_currency_unit(),
+            req.request.currency,
+            req.request.amount,
+            req,
+        ))?;
+        let connector_request = fiserv::FiservPaymentsRequest::try_from(&router_obj)?;
         let fiserv_payments_request = types::RequestBody::log_and_get_request_body(
             &connector_request,
             utils::Encode::<fiserv::FiservPaymentsRequest>::encode_to_string_of_json,
@@ -592,7 +608,13 @@ impl ConnectorIntegration<api::Execute, types::RefundsData, types::RefundsRespon
         &self,
         req: &types::RefundsRouterData<api::Execute>,
     ) -> CustomResult<Option<types::RequestBody>, errors::ConnectorError> {
-        let connector_request = fiserv::FiservRefundRequest::try_from(req)?;
+        let router_obj = fiserv::FiservRouterData::try_from((
+            &self.get_currency_unit(),
+            req.request.currency,
+            req.request.refund_amount,
+            req,
+        ))?;
+        let connector_request = fiserv::FiservRefundRequest::try_from(&router_obj)?;
         let fiserv_refund_request = types::RequestBody::log_and_get_request_body(
             &connector_request,
             utils::Encode::<fiserv::FiservRefundRequest>::encode_to_string_of_json,
diff --git a/crates/router/src/connector/fiserv/transformers.rs b/crates/router/src/connector/fiserv/transformers.rs
index ae8eed0af31..2d07da7f47a 100644
--- a/crates/router/src/connector/fiserv/transformers.rs
+++ b/crates/router/src/connector/fiserv/transformers.rs
@@ -9,6 +9,38 @@ use crate::{
     types::{self, api, storage::enums},
 };
 
+#[derive(Debug, Serialize)]
+pub struct FiservRouterData<T> {
+    pub amount: String,
+    pub router_data: T,
+}
+
+impl<T>
+    TryFrom<(
+        &types::api::CurrencyUnit,
+        types::storage::enums::Currency,
+        i64,
+        T,
+    )> for FiservRouterData<T>
+{
+    type Error = error_stack::Report<errors::ConnectorError>;
+
+    fn try_from(
+        (currency_unit, currency, amount, router_data): (
+            &types::api::CurrencyUnit,
+            types::storage::enums::Currency,
+            i64,
+            T,
+        ),
+    ) -> Result<Self, Self::Error> {
+        let amount = utils::get_amount_as_string(currency_unit, amount, currency)?;
+        Ok(Self {
+            amount,
+            router_data,
+        })
+    }
+}
+
 #[derive(Debug, Serialize, Eq, PartialEq)]
 #[serde(rename_all = "camelCase")]
 pub struct FiservPaymentsRequest {
@@ -99,23 +131,25 @@ pub enum TransactionInteractionPosConditionCode {
     CardNotPresentEcom,
 }
 
-impl TryFrom<&types::PaymentsAuthorizeRouterData> for FiservPaymentsRequest {
+impl TryFrom<&FiservRouterData<&types::PaymentsAuthorizeRouterData>> for FiservPaymentsRequest {
     type Error = error_stack::Report<errors::ConnectorError>;
-    fn try_from(item: &types::PaymentsAuthorizeRouterData) -> Result<Self, Self::Error> {
-        let auth: FiservAuthType = FiservAuthType::try_from(&item.connector_auth_type)?;
+    fn try_from(
+        item: &FiservRouterData<&types::PaymentsAuthorizeRouterData>,
+    ) -> Result<Self, Self::Error> {
+        let auth: FiservAuthType = FiservAuthType::try_from(&item.router_data.connector_auth_type)?;
         let amount = Amount {
-            total: utils::to_currency_base_unit(item.request.amount, item.request.currency)?,
-            currency: item.request.currency.to_string(),
+            total: item.amount.clone(),
+            currency: item.router_data.request.currency.to_string(),
         };
         let transaction_details = TransactionDetails {
             capture_flag: Some(matches!(
-                item.request.capture_method,
+                item.router_data.request.capture_method,
                 Some(enums::CaptureMethod::Automatic) | None
             )),
             reversal_reason_code: None,
-            merchant_transaction_id: item.connector_request_reference_id.clone(),
+            merchant_transaction_id: item.router_data.connector_request_reference_id.clone(),
         };
-        let metadata = item.get_connector_meta()?;
+        let metadata = item.router_data.get_connector_meta()?;
         let session: SessionObject = metadata
             .parse_value("SessionObject")
             .change_context(errors::ConnectorError::RequestEncodingFailed)?;
@@ -133,7 +167,7 @@ impl TryFrom<&types::PaymentsAuthorizeRouterData> for FiservPaymentsRequest {
             //card not present in online transaction
             pos_condition_code: TransactionInteractionPosConditionCode::CardNotPresentEcom,
         };
-        let source = match item.request.payment_method_data.clone() {
+        let source = match item.router_data.request.payment_method_data.clone() {
             api::PaymentMethodData::Card(ref ccard) => {
                 let card = CardData {
                     card_data: ccard.card_number.clone(),
@@ -389,35 +423,40 @@ pub struct SessionObject {
     pub terminal_id: String,
 }
 
-impl TryFrom<&types::PaymentsCaptureRouterData> for FiservCaptureRequest {
+impl TryFrom<&FiservRouterData<&types::PaymentsCaptureRouterData>> for FiservCaptureRequest {
     type Error = error_stack::Report<errors::ConnectorError>;
-    fn try_from(item: &types::PaymentsCaptureRouterData) -> Result<Self, Self::Error> {
-        let auth: FiservAuthType = FiservAuthType::try_from(&item.connector_auth_type)?;
+    fn try_from(
+        item: &FiservRouterData<&types::PaymentsCaptureRouterData>,
+    ) -> Result<Self, Self::Error> {
+        let auth: FiservAuthType = FiservAuthType::try_from(&item.router_data.connector_auth_type)?;
         let metadata = item
+            .router_data
             .connector_meta_data
             .clone()
             .ok_or(errors::ConnectorError::RequestEncodingFailed)?;
         let session: SessionObject = metadata
             .parse_value("SessionObject")
             .change_context(errors::ConnectorError::RequestEncodingFailed)?;
-        let amount =
-            utils::to_currency_base_unit(item.request.amount_to_capture, item.request.currency)?;
         Ok(Self {
             amount: Amount {
-                total: amount,
-                currency: item.request.currency.to_string(),
+                total: item.amount.clone(),
+                currency: item.router_data.request.currency.to_string(),
             },
             transaction_details: TransactionDetails {
                 capture_flag: Some(true),
                 reversal_reason_code: None,
-                merchant_transaction_id: item.connector_request_reference_id.clone(),
+                merchant_transaction_id: item.router_data.connector_request_reference_id.clone(),
             },
             merchant_details: MerchantDetails {
                 merchant_id: auth.merchant_account,
                 terminal_id: Some(session.terminal_id),
             },
             reference_transaction_details: ReferenceTransactionDetails {
-                reference_transaction_id: item.request.connector_transaction_id.to_string(),
+                reference_transaction_id: item
+                    .router_data
+                    .request
+                    .connector_transaction_id
+                    .to_string(),
             },
         })
     }
@@ -477,11 +516,14 @@ pub struct FiservRefundRequest {
     reference_transaction_details: ReferenceTransactionDetails,
 }
 
-impl<F> TryFrom<&types::RefundsRouterData<F>> for FiservRefundRequest {
+impl<F> TryFrom<&FiservRouterData<&types::RefundsRouterData<F>>> for FiservRefundRequest {
     type Error = error_stack::Report<errors::ConnectorError>;
-    fn try_from(item: &types::RefundsRouterData<F>) -> Result<Self, Self::Error> {
-        let auth: FiservAuthType = FiservAuthType::try_from(&item.connector_auth_type)?;
+    fn try_from(
+        item: &FiservRouterData<&types::RefundsRouterData<F>>,
+    ) -> Result<Self, Self::Error> {
+        let auth: FiservAuthType = FiservAuthType::try_from(&item.router_data.connector_auth_type)?;
         let metadata = item
+            .router_data
             .connector_meta_data
             .clone()
             .ok_or(errors::ConnectorError::RequestEncodingFailed)?;
@@ -490,18 +532,19 @@ impl<F> TryFrom<&types::RefundsRouterData<F>> for FiservRefundRequest {
             .change_context(errors::ConnectorError::RequestEncodingFailed)?;
         Ok(Self {
             amount: Amount {
-                total: utils::to_currency_base_unit(
-                    item.request.refund_amount,
-                    item.request.currency,
-                )?,
-                currency: item.request.currency.to_string(),
+                total: item.amount.clone(),
+                currency: item.router_data.request.currency.to_string(),
             },
             merchant_details: MerchantDetails {
                 merchant_id: auth.merchant_account,
                 terminal_id: Some(session.terminal_id),
             },
             reference_transaction_details: ReferenceTransactionDetails {
-                reference_transaction_id: item.request.connector_transaction_id.to_string(),
+                reference_transaction_id: item
+                    .router_data
+                    .request
+                    .connector_transaction_id
+                    .to_string(),
             },
         })
     }