🎨 Various changes

- Changed source additional data to dict
- Moved local cover saving into a manager
- Added stub for itch cover manager

src/store/managers/file_manager.py

@@ -8,5 +8,5 @@ class FileManager(AsyncManager):
 
     run_after = set((SteamAPIManager,))
 
-    def manager_logic(self, game: Game, _additional_data: tuple) -> None:
+    def manager_logic(self, game: Game, _additional_data: dict) -> None:
         game.save()