Handles the plain text version of the proxy protocol as defined in section 2.1: https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt

This module builds up the ProxyProtocol struct attributes as we read the PROXY line.

buffer represents the current state of the packet as it is being parsed

This implementation is designed to be monadic and exhaustive, if there is an error we continue to pass through each function until we unwrap at the end with the result.