Implement scope and file store management commands for cloud storage

From https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/dynamic-storage.html, we have the following additional features needed in various management commands. This assumes that #682 (closed) has been completed first.

• Add options to debusine-admin scope manage to allow modifying the set of file stores for a scope. (Note that this potentially includes enabling or disabling the shared local storage, which is a store.)
• Add a debusine-admin scope show command, showing data including a brief tabular representation of the contents of the scope's file stores. A file store with no files has either never been populated or has been drained, and can safely be removed.
• Add a debusine-admin delete_file_store command, mirroring the existing debusine-admin create_file_store. It must refuse to delete a file store that still contains files (unless --force is used), and suggest that the store be drained first. (!1614 (merged))

Report time in #539 (closed).