refactor(Core/InstanceScript): refactored load and save methods (#14977)

Co-authored-by: joschiwald <736792+joschiwald@users.noreply.github.com>

src/server/scripts/EasternKingdoms/Deadmines/deadmines.h

@@ -20,6 +20,7 @@
 
 #include "CreatureAIImpl.h"
 
+#define DataHeader "DM"
 #define DeadminesScriptName "instance_deadmines"
 
 enum DataTypes

src/server/scripts/EasternKingdoms/Deadmines/instance_deadmines.cpp

@@ -28,6 +28,7 @@ public:
     {
         instance_deadmines_InstanceMapScript(Map* map) : InstanceScript(map)
         {
+            SetHeaders(DataHeader);
         }
 
         void Initialize() override