add option to save as file instead of to db

[tybug]

when set, Cacher should cache replays to a file instead of as a blob inside the database.

Requires some thought for how it deals with replays where we only know the replay data (such as ReplayID) - should it put defaults / "we don't know this" values into osr fields like username, beatmap_hash, 300s, etc? Or should it cache to the db and ignore this option?

This will probably require us to implement osr reconstruction, ie creating an osr from all the necessary information. That's something that's good for an osu! utils library to have regardless though. Will likely require one or more api calls to get stuff like 300s/misses as we don't keep track of all that stuff currently.