Implement mod communication channels (#6351)

Implement network communication for channels

* Implement ModChannel manager server side to route incoming messages from clients to other clients
* Add signal handler switch on client & ModChannelMgr on client to handle channels
* Add Lua API bindings + client packet sending + unittests
* Implement server message sending
* Add callback from received message handler to Lua API using registration method

minetest.conf.example

@@ -1126,6 +1126,10 @@
 #    type: bool
 # enable_pvp = true
 
+#    Enable mod channels.
+#    type: bool
+# enable_mod_channels = false
+
 #    If this is set, players will always (re)spawn at the given position.
 #    type: string
 # static_spawnpoint =