Add Idempotency Support to Create Charge for On-Demand Subscriptions

Description:
The Create Charge endpoint for on-demand subscriptions currently does not support idempotency checks. As a result, multiple consecutive requests with the same payload can create multiple charges for the same customer.

This can lead to duplicate customer charges, particularly when a merchant retries a request because of a timeout, network failure, or a lost response after the initial request was successfully processed.

Expected Behavior:
The endpoint should support an idempotency key so that repeated requests with the same key are treated as the same operation. If the initial request has already been successfully processed, subsequent requests with the same idempotency key should return the result of the original request instead of creating another charge.

Use Case:
A merchant sends a Create Charge request, but the response is lost due to a network issue. The merchant retries the request with the same idempotency key. The retry should not create a second charge.

Impact:
Implementing idempotency would prevent accidental duplicate charges and make the Create Charge API safer and more reliable for merchants handling retries.