Support hiding instead of deleting for Backblaze B2 backend

I don't see a use case for this. Please explain.

B2 allows two methods of deletion: soft deletion (hiding) and full deletion (deleting). Full deletion is permanent and irrecoverable. Hiding simply creates a new version of the file, saying that the file has been hidden. Additionally, lifecycle rules can be set up on the B2 bucket to full delete hidden files after a certain number of days.

B2 application keys can be restricted to not allow full deletion, and only allow hiding. This means that even if the machine doing the backups is compromised, the backups can only be deleted by the lifecycle rules since the application key on the compromised machine does not have the ability to full-delete the backups.

For my use case, I'd like to be able to restrict the backup machine to only hide and write backups, and have B2 automatically delete hidden (soft deleted) backups after a certain number of days. By replacing the call to delete_file_version with hide_file, duplicity will hide files instead of fully deleting them, allowing them to be restored if required.

I've submitted a merge proposal here:

https://code.launchpad.net/~thomas-ross/duplicity/b2-hide-files/+merge/383164