Allow hosting pack on different port

[OffsetMonkey538]

Useful when the server is behind something like tcpshield, which filters out non-minecraft stuff.

[OffsetMonkey538]

Think simplest would be to, when the selected port is different than the minecraft one, start a separate netty server myself, which is bound to said port. This way I can easily just reuse the MainHttpHandler that already exists.

Not sure if this functionality is something I should bring to mesh lib? I mean, I think I'd have to modify it for doing what's above anyway, cause I need to have a way of removing/disabling the http handler given to meshlib