Issue:
feat(core): add payments session endpoint

## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [x] New feature

## Description
<!-- Describe your changes in detail -->
Add a new endpoint called session_tokens under payments. This will call the connectors and create sessions token.



## 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 its an obvious bug or documentation fix
that will have little conversation).
-->
Sessions token is used by wallets to initiate the wallet sdk.


## How did you test it?
NA


## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed submitted code


Resolution:
<think>
PR Context:
- Title: feat(core): add sessions api endpoint
- Merged: 2022-12-10T16:02:03Z
- Files changed: 30 files
- Commits: 30 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [x] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates

## Description
The PR's #100 and #95 should have been merged to main, but it was merged to [call_multiple_connectors](https://github.com/juspay/orca/tree/call_multiple_connectors) after it was merged to main.

## Motivation and Context
Please refer to #100 and #95


## How did you test it?
NA


## Checklist
<!-- Put an `x
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** feat(core): add sessions api endpoint (#104)
Files to modify:


**crates/router/src/configs/settings.rs**
  Add:
    - struct: pub::SupportedConnectors

**crates/router/src/core/payments.rs**
  Modify existing code

**crates/router/src/core/payments/helpers.rs**
  Add:
    - function: pub::get_connector_default

**crates/router/src/core/payments/operations.rs**
  Add:
    - function: private::get_connector
    - function: private::get_connector
    - function: private::get_connector
    - function: private::get_connector

**crates/router/src/core/payments/operations/payment_cancel.rs**
  Modify existing code

**crates/router/src/core/payments/operations/payment_capture.rs**
  Modify existing code

**crates/router/src/core/payments/operations/payment_confirm.rs**
  Modify existing code

**crates/router/src/core/payments/operations/payment_create.rs**
  Modify existing code

**crates/router/src/core/payments/operations/payment_method_validate.rs**
  Add:
    - function: private::get_connector

**crates/router/src/core/payments/operations/payment_response.rs**
  Add:
    - function: private::update_tracker
    - impl: impl PostUpdateTracker<F, PaymentData<F>, types::PaymentsSessionData> for PaymentResponse

**crates/router/src/core/payments/operations/payment_session.rs**
  Add:
    - function: private::get_connector

**crates/router/src/core/payments/operations/payment_start.rs**
  Add:
    - function: private::get_connector

**crates/router/src/core/payments/operations/payment_status.rs**
  Modify existing code

**crates/router/src/core/payments/operations/payment_update.rs**
  Modify existing code

**crates/router/src/core/payments/transformers.rs**
  Add:
    - function: private::generate_response
    - impl: impl ToResponse<Req, PaymentData<F>, Op> for api::PaymentsSessionResponse

**crates/router/src/core/refunds.rs**
  Modify existing code

**crates/router/src/routes/app.rs**
  Modify existing code

**crates/router/src/routes/payments.rs**
  Add:
    - function: pub::payments_connector_session

**crates/router/src/scheduler/workflows/payment_sync.rs**
  Modify existing code

**crates/router/src/schema.rs**
  Modify existing code

**crates/router/src/types.rs**
  Modify existing code

**crates/router/src/types/api.rs**
  Remove:
    - function: pub::construct
    - impl: impl ConnectorData
  Add:
    - enum: pub::ConnectorCallType
    - impl: impl ConnectorData

**crates/router/src/types/api/payments.rs**
  Add:
    - function: private::from
    - function: private::from
    - struct: pub::PaymentsSessionResponse
    - impl: impl From<PaymentsSessionRequest> for PaymentsResponse
    - impl: impl From<PaymentsSessionRequest> for PaymentsSessionResponse

**crates/router/src/types/storage/connector_response.rs**
  Modify existing code

**crates/router/src/types/storage/payment_attempt.rs**
  Modify existing code

**crates/router_derive/src/macros/operation.rs**
  Modify existing code

**crates/router_env/src/logger/types.rs**
  Modify existing code