Rework DefaultGraphConverter

[UnravelSports]

• We remove the old SoccerGraphConverter and rename SoccerGraphConverterPolars to SoccerGraphConverter
• We rework remove dangling files like default_player.py and default_frame.py that were only used by the old SoccerGraphConverter
• We move to_spektral_graphs, to_pickle, _verify_feature_funcs, return_dtypes and to_graph_frames to the DefaultGraphConverter so both AmericanFootballGraphConverter and SoccerGraphConverter can inherit straight from there.
• We align Column objects in the NFL dataset more closely to soccer such that we can ultimately (not yet done) move all functionality to DefaultGraphConverter. This means for example frameId and gameId are renamed to frame_id and game_id.