Support Idempotency-Key Header for Safety

The `Idempotency-Key` HTTP Header used by Stripe could be very useful to add as a `bweb` middleware. It can be used to prevent the same effect twice, meaning that you can send the same request twice and be sure that it only has the effect once. This is useful for when sending value and edge cases around failed requests.

https://stripe.com/blog/idempotency

Start by reading the Stripe idempotency article linked in the issue and then inspect bweb's middleware entry points. Define how the Idempotency-Key header should prevent repeated effects, including failed requests, and identify the middleware behavior and tests needed to demonstrate that duplicate requests are handled safely.

Written by the indexing model from the issue text.