Implement HAProxy PROXY protocol for relaying TCP information

[AlexanderThaller]

It would be usefull in environments where TLS is terminated on different machienes than the loadbalancer and client information (like the source IP) needs to be preserved.

See https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt for a description of the protocol.

I can try to implement it as I have some rust experience but I never worked with tokio directly so I have to see.