Implement support for multipath

[djc]

QUIC v1 will not include support for multipath traffic, but it is already being discussed. In particular, there's an Internet Draft here:

https://datatracker.ietf.org/doc/draft-deconinck-quic-multipath/?include_text=1

This issue will help us keep track of implementation efforts of multipath in Quinn.

[link2xt]

Multipath QUIC website: https://multipath-quic.org/
GitHub repository: https://github.com/quicwg/multipath
IETF draft: https://datatracker.ietf.org/doc/draft-ietf-quic-multipath/

[dignifiedquire]

Some interesting background on the various forms of the current proposals: https://www.privateoctopus.com/2024/02/28/long-slow-path-to-quic-multipath.html

[flub]

FWIW we've started working on this over at n0-computer#28

It'll be a while before that becomes anything useful, but let's see how progress is. The intention is of course to upstream it once it becomes usable.