todo: Optimise encode process for signature based requests

### Feature Description

For a connector call, `get_request_body` is being called twice, Once in `get_headers`, other in `build_request`.

This is an expensive operation to call the same encoding function twice.

### Possible Implementation

Need to optimise it to a state from where it can be further implemented into the methods.

### 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/contrib/CONTRIBUTING.md)

### Are you willing to submit a PR?

None

Start by tracing the connector request flow around get_request_body, get_headers, and build_request to confirm where the encoding work is repeated. Determine a state or shared result that supports both methods without encoding twice; done means the request path performs one encoding operation and existing connector behavior remains covered by tests.

Written by the indexing model from the issue text.