String returned by a specific storage implementation to identify a backup, probably a folder name
which is exactly the same with the backup name we pass into
This is created and returned by the storage when
create_backup(), passed back to the storage
create_for_write() and persisted nowhere (once a backup is created, files are referred to
URI pointing to a file in a backup storage, like "s3:///bucket/path/file".
These are created by the storage when
create_for_write(), stored in manifests by the backup
controller, and passed back to the storage when
open_for_read() by the restore controller
to retrieve a file referred to in the manifest.