HTTP: support generating fake PROXY protocol header

[slandelle]

Some applications might require a fake PROXY protocol header to perform properly based on the information in there, in particular the source address and port.

https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt

In this ticket, we're going to support the following:

• only for HTTP protocol support
• header version 1 only
• INET protocol and family: IPv4 only, header won't be generated if the resolution results in an IPv6 address
• source address and port: fetched from an Expression[InetSocketAddress] so the user can define his own generators and typically generate InetSocketAddress.createUnresolved(hostname, port)
• destination address: the one of the proxy
• destination port: the one of the proxy

The only thing the user will have to do to enable this feature will be to add enableFakeProxyProtocolHeader() on the HttpProtocol configuration and the source InetSocketAddress generator.