Direct support for `Stripe-Account` and `Idempotency-Key` headers

[tysonzero]

Not particularly pressing as configSecurityScheme is a simple enough workaround for inserting arbitrary headers, but it'd be nice to have these formally supported.

[joel-bach]

I guess this means you already have an implementation for this, right? Feel free to open a pull request with the necessary changes, I would appreciate it. Thanks!

[tysonzero]

How should I structure it to be most friendly to future runs of the code generator?

[joel-bach]

I would suggest to create a SecuritySchemes.Extra module which contains this additional security scheme and then list it in the README.