Add support for Proxy Protocol

[ColinHebert]

If the connection to endlessh is done via a proxy (say Traefik over TCP), the IP detected will be the one of the proxy.

Fortunately, the Proxy Protocol allows to pass some additional headers in TCP to track the original IP address.

Here's an example of why Proxy Protocol can be helpful.

This is a fairly popular wrapper which allows the use of Proxy Protocol