Core/Players: Implemented item level based armor damage reduction diminishing for pvp

Port From (https://github.com/TrinityCore/TrinityCore/commit/255082782cef48f68ec4ad041202be8e8693071c)

Source/Game/DataStorage/Structs/GameTablesRecords.cs

@@ -92,6 +92,11 @@ namespace Game.DataStorage
         public float Health;
     }
 
+    public sealed class GtItemLevelByLevelRecord
+    {
+        public float ItemLevel;
+    }
+
     public sealed class GtItemSocketCostPerLevelRecord
     {
         public float SocketCost;