Add X-Rate-Limit headers to mint responses

[Egge21M]

As rate limiting access to mint endpoints is a very common practice it might make sense to communicate these limits clearly to API consumers. This makes sure that wallets and services do not run into these limits by accident.

While there is an active draft to standardise these headers, as of right now there is only the following defacto standard:

X-Rate-Limit-Limit - the rate limit ceiling that is applicable for the current request.
X-Rate-Limit-Remaining - the number of requests left for the current rate-limit window.
X-Rate-Limit-Reset - the time at which the rate limit resets, specified in UTC epoch time (in seconds).

[ok300]

A good reference, with examples, is: https://developer.okta.com/docs/reference/rl-best-practices/