"Repair" option for game caches

[yungcomputerchair]

Since the game client itself does no hash checking or validation, it is not able to detect when a "complete" (accompanied by an __info file) asset bundle is corrupt.

We initially decided that having a game cache repair feature was out of scope because the work that would be needed to identify the compressed bundle for a game cache file, download that bundle, then extract it, would be non-trivial.

However, the game client will redownload files that are missing or not "complete", meaning there is a way forward of just deleting the files that were identified by ffbuildtool as corrupt and allowing the client to download fresh copies.

The implementation here should be relatively simple. The frontend already contains a list of all game cache files along with flags for corrupted/missing. We could have a new API that passes a list of the corrupted files to the backend and then delete them with std::fs.