feat: add prefix flag to migrator backup commands

[ianroberts]

Add a flag to the migrator command to allow the user to specify a key prefix that they want to back up, rather than hard coding to always back up the entire database. This is useful in several situations, including:

• when a user wants to back up only specific object types, e.g. to back up just their secrets they could specify --prefix /registry/secrets
• when they have an older dqlite database created by microk8s v1.26 or earlier, and are unable to back up at all due to issue k8s-dqlite migrator exits immediately with code 0, no backup created #341 - in this case passing --prefix /registry will allow the backup to proceed correctly.

If the new flag is not supplied, the default behaviour matches the current hard-coded behaviour for backwards compatibility - an empty prefix "" for backup-etcd and the prefix "/" for backup-dqlite.

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