Support PROXY protocol when running behind Load Balancer

[aler9]

Discussed in https://github.com/aler9/rtsp-simple-server/discussions/477

^{Originally posted by dorinclisu July 27, 2021} Is it difficult to support it? https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt

The reason is, behind a load balancer all rtsp server request IP's are of the balancer itself, therefore losing the ability to whitelist client IP's.

The load balancer I'm using does have a proxy toggle, and specifically warns it that the application must support it if enabled. I did try it on and the request wouldn't reach the rtsp server anymore so I assume it doesn't support it yet.