Use standard idempotency key header

[nickevansuk]

Idempotency is currently achieved for B using the UUID, however a more standard approach should be adopted to ease implementation with existing libraries and frameworks.

https://datatracker.ietf.org/doc/draft-ietf-httpapi-idempotency-key-header/

If this is still in draft by the time it is considered for inclusion in the spec, suggest using x-idempotency-key in line with Open Banking, rather than using the draft before it is ratified.

[nathansalter]

I'd recommend just using the actual header. Migrating header names can be a pain, and it's recommended to not use the X- prefix any more.