nixos/snapserver: init

A nixos module for configuring the server side of pkgs.snapcast.
The module is named "snapserver" following upstream convention.
This commit does not provide module for the corresponding client.

Fix handling of port and controlPort

Fix stream uri generation & address review

Remove unused streams options & add description

Add missing description & Remove default fs path

Use types.port for ports & formatting improvements

Force mpd and mopidy to wait for snapserver