Core/Spells: Added custom spell cast arg

Port From (https://github.com/TrinityCore/TrinityCore/commit/1c52c31274db5ba1b796eacb6ddd87309cc02333)

Source/Game/Entities/Object/WorldObject.cs

@@ -2428,6 +2428,8 @@ namespace Game.Entities
                 }
             }
 
+            spell.m_customArg = args.CustomArg;
+
             return spell.Prepare(targets.Targets, args.TriggeringAura);
         }
 

Source/Game/Spells/Spell.cs

@@ -7968,6 +7968,7 @@ namespace Game.Spells
         public bool m_fromClient;
         public SpellCastFlagsEx m_castFlagsEx;
         public SpellMisc m_misc;
+        public object m_customArg;
         public SpellCastVisual m_SpellVisual;
         public SpellCastTargets m_targets;
         public sbyte m_comboPointGain;
@@ -9202,6 +9203,7 @@ namespace Game.Spells
         public ObjectGuid OriginalCastId = ObjectGuid.Empty;
         public int? OriginalCastItemLevel;
         public Dictionary<SpellValueMod, int> SpellValueOverrides = new();
+        public object CustomArg;
 
         public CastSpellExtraArgs() { }
 
@@ -9298,6 +9300,12 @@ namespace Game.Spells
             SpellValueOverrides.Add(mod, val);
             return this;
         }
+
+        public CastSpellExtraArgs SetCustomArg(object customArg)
+        {
+            CustomArg = customArg;
+            return this;
+        }
     }
 
     public class SpellLogEffect