Add borg compact command support

This addresses a novelty of borg 1.2 which now includes a compact command. See #11351

Note that I didn't try to make it smart. One may set compact to yes explicitely. No attempt at detect whether remote will support it.

Also, didn't activate it by default, as upstream doesn't seem to recommend it: https://borgbackup.readthedocs.io/en/stable/usage/notes.html#separate-compaction