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

builtin/client/register.lua

@@ -71,3 +71,5 @@ core.registered_on_dignode, core.register_on_dignode = make_registration()
 core.registered_on_punchnode, core.register_on_punchnode = make_registration()
 core.registered_on_placenode, core.register_on_placenode = make_registration()
 core.registered_on_item_use, core.register_on_item_use = make_registration()
+core.registered_on_modchannel_message, core.register_on_modchannel_message = make_registration()
+core.registered_on_modchannel_signal, core.register_on_modchannel_signal = make_registration()

builtin/game/register.lua

@@ -583,6 +583,7 @@ core.registered_on_punchplayers, core.register_on_punchplayer = make_registratio
 core.registered_on_priv_grant, core.register_on_priv_grant = make_registration()
 core.registered_on_priv_revoke, core.register_on_priv_revoke = make_registration()
 core.registered_can_bypass_userlimit, core.register_can_bypass_userlimit = make_registration()
+core.registered_on_modchannel_message, core.register_on_modchannel_message = make_registration()
 
 --
 -- Compatibility for on_mapgen_init()

builtin/settingtypes.txt

@@ -928,6 +928,9 @@ player_transfer_distance (Player transfer distance) int 0
 #    Whether to allow players to damage and kill each other.
 enable_pvp (Player versus Player) bool true
 
+#    Enable mod channels support.
+enable_mod_channels (Mod channels) bool false
+
 #    If this is set, players will always (re)spawn at the given position.
 static_spawnpoint (Static spawnpoint) string