Implement proxy protocol support for TCP inputs

## What?
Similar to https://github.com/elastic/logstash/issues/4418, it would be nice if Graylog would support proxy protocol on TCP inputs.
https://www.haproxy.com/blog/use-the-proxy-protocol-to-preserve-a-clients-ip-address/
https://github.com/haproxy/haproxy/blob/master/doc/proxy-protocol.txt

## Why?
Proxy protocol allows preserving the original client IP when traffic passes through a reverse proxy/load balancer.
For our use case, this would allow us to deal with TLS termination outside of Graylog to simplify TLS deployment and certificate updates.

## Your Environment
* Graylog Version: 4.3.3
* Elasticsearch Version: N/A
* MongoDB Version: N/A
* Operating System: N/A
* Browser version: N/A