Core/Spells: Implemented evoker empower spell mechanic

Port From (https://github.com/TrinityCore/TrinityCore/commit/a39d0db9ec64f6bf38716abaade5b7835f2db338)

Source/Game/Entities/Player/Player.Fields.cs

@@ -112,6 +112,7 @@ namespace Game.Entities
         ObjectGuid m_temporaryUnsummonedBattlePet;
         Dictionary<uint, StoredAuraTeleportLocation> m_storedAuraTeleportLocations = new();
         SpellCastRequest _pendingSpellCastRequest;
+        float m_empowerMinHoldStagePercent;
 
         //Mail
         List<Mail> m_mail = new();