Provide s2n-quic as a backend for HTTP/3

[seanmonstar]

The HTTP/3 feature in reqwest uses h3, which can plug in QUIC libraries. In-tree, there currently is support for Quinn. It could be useful to provide s2n-quic as an alternative backend.

[seanmonstar]

There's s2n-quic-h3, but it's not published. One possibility is vendoring the code, since it's mostly glue between h3 and s2n-quic.

@camshaft do you think this would be easy-ish for someone to add, that could integrate with rustls?

[camshaft]

I think we should be able to get s2n-quic-h3 published, probably trying to match the H3 crate version as much as possible. But after that is published I don't think it would be that much work to get integrated. That being said, I haven't looked a lot at the places where Quinn was integrated so take it with a grain of salt 🙂