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

src/CMakeLists.txt

@@ -420,6 +420,7 @@ set(common_SRCS
 	mg_decoration.cpp
 	mg_ore.cpp
 	mg_schematic.cpp
+	modchannels.cpp
 	mods.cpp
 	nameidmapping.cpp
 	nodedef.cpp