"PROXY_PROTO" for L4 QUIC proxying

It's nice to be able to L4 proxy QUIC without losing original client IP.
If you can send large packets, one solution is to prepend a proxy-proto style header from "maglev" style Enovy to L7 Envoy.

One way to signal if a packet is munged by setting the version from standard QUIC versions to something synthetic like kPROX. I'd be happy to sanitize some of our munging and unmunging code when/if someone takes a look at this.