snapserver: add tcp input port forward option for incoming tcp streams.

[tocklime]

You can add an input stream to snapserver with a stream like:

- 'tcp://0.0.0.0:4953/?name=tcp_in'

But this being docker, we'd need a port forward into the container to see it.

With this in place, I reckon I'll be able to have a tannoy/public address like system. (especially with the meta:/// stream type coming in snapcast 0.22)