Feature request - "force" option to override existing snapshots or backups in destination

In case an existing subvolume with similar name exists in backup or snapshot destination, btrbk fails claiming for stray subvolume.

As reported in [https://github.com/digint/btrbk/issues/282](https://github.com/digint/btrbk/issues/282) there are some use cases where btrbk clean will not clean those existing subvolumes.

Proposal is to add a "force" option that could override the existing stray subvolume (by issuing a btrfs subvolume delete before creating it).

Start by tracing the code path that reports a stray subvolume during backup or snapshot creation, then inspect how btrbk clean handles the cases discussed in issue #282. Define where the force option is accepted and ensure it deletes the existing destination subvolume before creation; done means the override works without force retaining the current safety behavior.

Written by the indexing model from the issue text.