[DO NOT MERGE] quic git

[kim]

This implements a very simple git HTTP/3 server and client transport using custom TLS certificate validation rules (basically, a self-signed cert using the device key is presented). See inline commentary for pointers where we're not there yet.

One major peculiarity is registering the transport with libgit, which needs to happen only once. We will need to rework the library such that it forces the user to run some initialisation logic.

I also thought it might be overall the most straightforward to just use HTTP/3 instead of a custom protocol over plain QUIC, but I'm not sure how smart this is. Basically, my thoughts were: ALPN (application-level protocol negotiation) happens on the connection level, not streams. So, if we negotiate a custom protocol, it will be a bit tricky to "upgrade" an individual stream to HTTP/3 if so desired. Otoh, we could just use HTTP/3 and use normal request matching to dispatch different protocol concerns. This feels a bit clunky, however. Specifically, support for streaming request bodies seems a bit tricky due to having to switch between synchronous (libgit transport) and asynchronous code (quinn).

Let me know what you think.

[kim]

Uhm yeah, the e2e test fails on CI. It works locally though, check it out.

[kim]

Superseded by #70