WIP: Add HTTP/3 support

[seanmonstar]

This starts HTTP/3 support, making use of the traits in #3924, and integrating h3 into hyper::proto::h3.

It is not complete or working. I have divided attention, but figured I'd publish what I have working so far, in an effort to allow others to add on top of it. I'll keep this as a draft until it's close to working.

To make the diff easier to see, I'm making the target the quic branch. Note to self: switch it back before merging.

cc #1818

[Rafael-Conde]

@seanmonstar Hi there!

I've followed the http3 support discussions, and end up here. I've been thinking about contributing.

TBH, I haven't done an implementation like this, but I've seen in the PR that you introduced the impl blocks with todo!()s in the implementations.

I think I could study the architecture and maybe I could pull off some implementation, with your review, something useful might come out of it.

Do you have any plans for the next step so I could try to work in sync with you?

I'll soon make an attempt at it

[seanmonstar]

@Rafael-Conde I don't personally have plans to focus on this PR at least this year, but if you'd like to try to make progress, you're welcome :)

[Rafael-Conde]

@seanmonstar Ok then! Can I iterate in the implementation with your feedback reviewing it?

I'll likely fall into pitfalls due to the lack of experience in this area, but I'll try my best here.