Enable option to maintain & provide specific backups to restore FossilDB via FossilDB-client

[aaronkanzer]

Detailed Description

While standing up our own self-hosted version of WebKNOSSOS, we have been looking into data integrity/site reliability -- mostly, how to efficiently backup and restore our systems to a certain point in time.

Via the fossildb-client, the backup and restore commands work as intended -- as provided here; however, upon backup, we observe that the most recent backup is overwritten, and that no option is provided for where to point a new restore command (if multiple backups existed on the file system)

Specifically for the restore command, further logic could be available upon the protobuf extension of RestoreFromBackupRequest

proto_rpc.FossilDBStub(channel).RestoreFromBackup(proto.RestoreFromBackupRequest())

While this is not a blocker (we intend to routinely export the data off the EC2 instance to S3, etc.), it could become a useful convenience for users, and to quickly get systems back online if multiple backups can live next door to running instances.

Let us know -- we are happy to attempt a PR if this is something that could be useful -- otherwise, if you have insight into how you rountinely backup and restore FossilDB instances, that would be much appreciated!

Cc @kabilar