Rate limiting headers middleware

[baywet]

Creating this issue on a very good suggestion from @jansenbe.

With the rate limiting RFC reaching maturity and the existing headers sent back by identity services we could either augment the retry handler or add a new handler all together which would use that information to artificially slow down requests based on a quota consumption percentage. The rationale is that cooling down before hitting the ceiling usually gives you a better throughput than hitting the ceiling and waiting for the retry after time (some services penalize you for eating up all your quota).