Support idempotent requests

[waiting-for-dev]

Desired Behavior

Stripe API supports sending idempotent requests to avoid sending the same request twice in case of connectivity errors or users double-clicking by error.

It would be nice to support the feature from solidus_stripe.

(Optional) Additional notes

That would probably require an upstream change or creating a custom middleware, as the idempotency key is sent as a request header.