Support PROXY protocol (v1/v2)

[aquarapid]

Haproxy pioneered a protocol to communicate client connection details (source IP/port) from the SLB layer to the service behind the SLB. It would be great if vtgate could support this, and thus log the actual client connection details, since vtgate is typically deployed behind an SLB.

There is a go module out there that implements this: https://github.com/pires/go-proxyproto

Page about the protocol (high level, including existing software that supports it): https://www.haproxy.com/blog/haproxy/proxy-protocol/

Spec here: https://www.haproxy.org/download/2.0/doc/proxy-protocol.txt

[aquarapid]

Fixed by #5563