Core/Creatures: Allow overriding non-gossip based trainer ids for creatures

Port From (https://github.com/TrinityCore/TrinityCore/commit/689d3a667af8c3e2662b77d14edf201771130bee)

Source/Game/Entities/Creature/Creature.Fields.cs

@@ -27,6 +27,8 @@ namespace Game.Entities
 
         bool _isMissingCanSwimFlagOutOfCombat;
 
+        uint? _trainerId;
+
         public ulong m_PlayerDamageReq;
         public float m_SightDistance;
         public float m_CombatDistance;

Source/Game/Entities/Creature/Creature.cs

@@ -1129,6 +1129,19 @@ namespace Game.Entities
             SetHomePosition(GetPosition());
         }
 
+        public uint GetTrainerId()
+        {
+            if (_trainerId.HasValue)
+                return _trainerId.Value;
+
+            return Global.ObjectMgr.GetCreatureDefaultTrainer(GetEntry());
+        }
+
+        public void SetTrainerId(uint? trainerId)
+        {
+            _trainerId = trainerId;
+        }
+        
         public override bool IsMovementPreventedByCasting()
         {
             // first check if currently a movement allowed channel is active and we're not casting

Source/Game/Handlers/NPCHandler.cs

@@ -52,7 +52,7 @@ namespace Game
                 return;
             }
 
-            uint trainerId = Global.ObjectMgr.GetCreatureDefaultTrainer(npc.GetEntry());
+            uint trainerId = npc.GetTrainerId();
             if (trainerId != 0)
                 SendTrainerList(npc, trainerId);
             else