feat: getter for the used instance IDs (#14677)

* feat: getter for the used instance IDs

* doxygen

src/server/game/Maps/MapMgr.h

@@ -128,6 +128,15 @@ public:
         return std::fmod(o, 2.0f * static_cast<float>(M_PI));
     }
 
+    /**
+    * @name GetInstanceIDs
+    * @return vector of instance IDs
+    */
+    std::vector<bool> GetInstanceIDs()
+    {
+        return _instanceIds;
+    }
+
     void DoDelayedMovesAndRemoves();
 
     Map::EnterState PlayerCannotEnter(uint32 mapid, Player* player, bool loginCheck = false);