Add support for persistent H3 connections.

[bwelling]

No description provided.

[bwelling]

It might be worth marking this as beta, or subject to change in the future, as if httpx ever gains h3 support, then we'd want to use that (for consistency with h1 and h2) rather than aioquic.

[bwelling]

I'm not sure why pyright is complaining here, as the code is basically identical to that in the quic methods, which it doesn't complain about.

[rthalley]

pyright (and mypy) don't always figure out non-trivial dependency arrangements. Also, the quic code works because it has # pyright: ignore on the assignment. (which is bad as there is no timeout parameter for make_stream in the sync case, which we miss!)

Probably the better way to do this is to declare the_connection to be Optional[dns.quic.SyncQuicConnection], initialize it to None. It will probably figure out that it's always None, but if not (or if mypy doesn't) then assert this_connection is not None right before use will fix it.