Implementation of
https://datatracker.ietf.org/doc/draft-ietf-ippm-responsiveness/ (draft
5) with flexible download and upload handlers to suit other use cases as
well.
### Motivation:
The provided handlers are useful for measuring responsiveness and
testing things like performance of proxies
### Modifications:
Add `NIOHTTPResponsiveness` and `NIOHTTPResponsivenessServer`
### Result:
We'll now have an implementation of the Responsiveness under Working
Conditions draft
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Co-authored-by: George Barnett <gbarnett@apple.com>
