#145 Follow-Up: Implement Garbage Collection for unused catalog images in the cache

[everettraven]

Following #145 we should add some form of garbage collection that is able to evaluate what catalog images (and SHAs) are no longer used and clean them up. This will prevent us from consuming an unbounded amount of disk space in the filesystem cache.

Acceptance Criteria:

• Garbage collection is implemented for the ImageRegistry source.

[joelanford]

Potential source of inspiration: operator-framework/rukpak@main/internal/uploadmgr/gc.go