Proxy protocol v2 support for ingress and egress

**Is your feature request related to a problem? Please describe.**
We currently have client IP propagated down (over multiple proxies) to the RMQ instance. If we were to use amqpprox and retain the same functionality, we would need to have support for ingress proxy protocol on amqpprox side as well, for we can't position amqpprox on the edge. Due to the fact that our infrastructure supports only proxy protocol v2... well, that's why we would like to see in/out v2 support. 😃

**Describe the solution you'd like**
Implementation of proxy protocol on ingress so that amqpproxy could propagate the real client IP down the road... and while on it we would like to see support for v2 proxy protocol thrown in. 😄

**Describe alternatives you've considered**
We could forget about real client IPs, but that is unfortunately easier said than done. 🤷🏻‍♂️

The issue requests proxy protocol v2 support on both ingress and egress but names no files, tests, or entry points. First locate the connection-handling paths and existing proxy-protocol support, then trace how client IP metadata is propagated. Done means amqpprox accepts and emits v2 headers while preserving the real client IP across the proxy.

Written by the indexing model from the issue text.