Issue:
Bug: [FEATURE] add domain type for client secret 



### Feature Description

Create domain type for client secret

### Possible Implementation

Make type `ClietSecret(String)` and use it wherever applicable

### Have you spent some time to check if this feature request has been raised before?

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

### Have you read the Contributing Guidelines?

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

### Are you willing to submit a PR?

No, but I'm happy to collaborate on a PR with someone else

Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [x] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description

Implement `ClientSecret` type for payments. 

Closes #1357 

### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables


## Motivation and Context


## How did you test it?
Added and ran unit tests successfully.

_Maintainer edit:_ This PR only adds a newtype and is not being used anywhere in code, this should not affect any existing application behavior.

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [x] I added unit tests for my changes where possible
- [ ] I added a [CHANGELOG](/CHANGELOG.md) entry if applicable


Testing:
Added and ran unit tests successfully.

_Maintainer edit:_ This PR only adds a newtype and is not being used anywhere in code, this should not affect any existing application behavior.

Patch:
diff --git a/crates/api_models/src/payments.rs b/crates/api_models/src/payments.rs
index 6bccfd1124c..1be9762c76c 100644
--- a/crates/api_models/src/payments.rs
+++ b/crates/api_models/src/payments.rs
@@ -1,4 +1,4 @@
-use std::num::NonZeroI64;
+use std::{fmt, num::NonZeroI64};
 
 use cards::CardNumber;
 use common_utils::{
@@ -8,6 +8,10 @@ use common_utils::{
 };
 use masking::Secret;
 use router_derive::Setter;
+use serde::{
+    de::{self, Unexpected, Visitor},
+    Deserialize, Deserializer, Serialize, Serializer,
+};
 use time::PrimitiveDateTime;
 use url::Url;
 use utoipa::ToSchema;
@@ -51,6 +55,120 @@ pub struct BankCodeResponse {
     pub eligible_connectors: Vec<String>,
 }
 
+#[derive(Debug, PartialEq)]
+pub struct ClientSecret {
+    pub payment_id: String,
+    pub secret: String,
+}
+
+impl<'de> Deserialize<'de> for ClientSecret {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct ClientSecretVisitor;
+
+        impl<'de> Visitor<'de> for ClientSecretVisitor {
+            type Value = ClientSecret;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+                formatter.write_str("a string in the format '{payment_id}_secret_{secret}'")
+            }
+
+            fn visit_str<E>(self, value: &str) -> Result<ClientSecret, E>
+            where
+                E: de::Error,
+            {
+                let (payment_id, secret) = value.rsplit_once("_secret_").ok_or_else(|| {
+                    E::invalid_value(Unexpected::Str(value), &"a string with '_secret_'")
+                })?;
+
+                Ok(ClientSecret {
+                    payment_id: payment_id.to_owned(),
+                    secret: secret.to_owned(),
+                })
+            }
+        }
+
+        deserializer.deserialize_str(ClientSecretVisitor)
+    }
+}
+
+impl Serialize for ClientSecret {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let combined = format!("{}_secret_{}", self.payment_id, self.secret);
+        serializer.serialize_str(&combined)
+    }
+}
+
+#[cfg(test)]
+mod client_secret_tests {
+    #![allow(clippy::expect_used)]
+
+    use serde_json;
+
+    use super::*;
+
+    #[test]
+    fn test_serialize_client_secret() {
+        let client_secret1 = ClientSecret {
+            payment_id: "pay_3TgelAms4RQec8xSStjF".to_string(),
+            secret: "fc34taHLw1ekPgNh92qr".to_string(),
+        };
+        let client_secret2 = ClientSecret {
+            payment_id: "pay_3Tgel__Ams4RQ_secret_ec8xSStjF".to_string(),
+            secret: "fc34taHLw1ekPgNh92qr".to_string(),
+        };
+
+        let expected_str1 = r#""pay_3TgelAms4RQec8xSStjF_secret_fc34taHLw1ekPgNh92qr""#;
+        let expected_str2 = r#""pay_3Tgel__Ams4RQ_secret_ec8xSStjF_secret_fc34taHLw1ekPgNh92qr""#;
+
+        let actual_str1 =
+            serde_json::to_string(&client_secret1).expect("Failed to serialize client_secret1");
+        let actual_str2 =
+            serde_json::to_string(&client_secret2).expect("Failed to serialize client_secret2");
+
+        assert_eq!(expected_str1, actual_str1);
+        assert_eq!(expected_str2, actual_str2);
+    }
+
+    #[test]
+    fn test_deserialize_client_secret() {
+        let client_secret_str1 = r#""pay_3TgelAms4RQec8xSStjF_secret_fc34taHLw1ekPgNh92qr""#;
+        let client_secret_str2 =
+            r#""pay_3Tgel__Ams4RQ_secret_ec8xSStjF_secret_fc34taHLw1ekPgNh92qr""#;
+        let client_secret_str3 =
+            r#""pay_3Tgel__Ams4RQ_secret_ec8xSStjF_secret__secret_fc34taHLw1ekPgNh92qr""#;
+
+        let expected1 = ClientSecret {
+            payment_id: "pay_3TgelAms4RQec8xSStjF".to_string(),
+            secret: "fc34taHLw1ekPgNh92qr".to_string(),
+        };
+        let expected2 = ClientSecret {
+            payment_id: "pay_3Tgel__Ams4RQ_secret_ec8xSStjF".to_string(),
+            secret: "fc34taHLw1ekPgNh92qr".to_string(),
+        };
+        let expected3 = ClientSecret {
+            payment_id: "pay_3Tgel__Ams4RQ_secret_ec8xSStjF_secret_".to_string(),
+            secret: "fc34taHLw1ekPgNh92qr".to_string(),
+        };
+
+        let actual1: ClientSecret = serde_json::from_str(client_secret_str1)
+            .expect("Failed to deserialize client_secret_str1");
+        let actual2: ClientSecret = serde_json::from_str(client_secret_str2)
+            .expect("Failed to deserialize client_secret_str2");
+        let actual3: ClientSecret = serde_json::from_str(client_secret_str3)
+            .expect("Failed to deserialize client_secret_str3");
+
+        assert_eq!(expected1, actual1);
+        assert_eq!(expected2, actual2);
+        assert_eq!(expected3, actual3);
+    }
+}
+
 #[derive(Default, Debug, serde::Deserialize, serde::Serialize, Clone, ToSchema)]
 pub struct CustomerDetails {
     /// The identifier for the customer.