feat(ScriptMgr): add OnArenaStart hook (#19922)

src/server/game/Scripting/ScriptDefines/ArenaScript.h

@@ -28,6 +28,7 @@ enum ArenaHook
     ARENAHOOK_ON_GET_POINTS,
     ARENAHOOK_CAN_SAVE_TO_DB,
     ARENAHOOK_ON_BEFORE_CHECK_WIN_CONDITION,
+    ARENAHOOK_ON_ARENA_START,
     ARENAHOOK_END
 };
 
@@ -48,6 +49,8 @@ public:
     [[nodiscard]] virtual bool OnBeforeArenaCheckWinConditions(Battleground* const /* bg */) { return true; }
 
     [[nodiscard]] virtual bool CanSaveToDB(ArenaTeam* /*team*/) { return true; }
+
+    virtual void OnArenaStart(Battleground* /* bg */) { };
 };
 
 #endif