Support HTTP/2 (and possibly HTTP/3 & QUIC) in proxy extension

As discussed in the generic topic Proxy extension and HTTP/2, the proxy extension currently only supports HTTP/1.x in back-end communication.

Support for HTTP/2 (and ideally HTTP/3 w/ QUIC transport as well) could increase throughput and improve general performance for reverse proxy configurations. Only requirement would be that the back-end is configured with HTTPS and (naturally) supports HTTP/2+, and ALPN handles the rest.