[Remote Store] Garbage Collection for Remote Backed Storage

[psychbot]

Is your feature request related to a problem? Please describe.
As part of #8201 we are moving stale commit deletion to asynchronous flow. Its possible that there can be files which doesn't get deleted from remote store due asynchronous nature.

Eg - If the deletion fails while closing an index this might lead to lingering files present in S3.

Describe the solution you'd like
A background time based job which will try to keep only the files which are referenced locally and delete anything which is not referenced from remote store.