Add an on_attack callback for mobs (#4064)

Added an on_attack callback that allows to execute additional custom logic after each attack.

Reviewed-on: https://git.minetest.land/MineClone2/MineClone2/pulls/4064
Co-authored-by: the-real-herowl <wiktor_t-i@proton.me>
Co-committed-by: the-real-herowl <wiktor_t-i@proton.me>

mods/ENTITIES/mcl_mobs/combat.lua

@@ -1252,5 +1252,9 @@ function mob_class:do_states_attack (dtime)
 		self.attack_state(self, dtime)
 	else
 
+	if self.on_attack then
+		self.on_attack(self, dtime)
+	end
+
 	end
 end

mods/ENTITIES/mcl_mobs/init.lua

@@ -314,7 +314,8 @@ function mcl_mobs.register_mob(name, def)
 
 			return self:mob_activate(staticdata, def, dtime)
 		end,
-		attack_state = def.attack_state,
+		attack_state = def.attack_state, -- custom attack state
+		on_attack = def.on_attack, -- called after attack, useful with otherwise predefined attack states (not custom)
 		harmed_by_heal = def.harmed_by_heal,
 		is_boss = def.is_boss,
 		dealt_effect = def.dealt_effect,