Test blobstore backup in simulation

The backup and restore simulation tests only use file backups. This all of the code related to using blobstore is untested in simulation.

The project would implement a very simple blobstore in flow so that we can test blob based backup and restore in simulation.

Start by locating the simulation backup and restore tests and the existing file-backup path, then trace where blobstore-backed operations would enter the simulation. Done means simulation covers blobstore backup and restore behavior rather than only file backups.

Written by the indexing model from the issue text.