Support UDP proxy protocol

[sando38]

If we deploy eturnal in managed kubernetes environments, we require a loadbalancer service, which forwards the traffic to eturnal. These traffic will not have the real IP address of the connecting clients but from the loadbalancer. Loadbalancer usually support proxy protocol though, therefore, having proxy protocol for UDP transports would solve the problem.

Nodeport is not a suitable solution, because we will always lose the IP addresses here as well, because the connecting IP address will be the kubernetes local gateway.