[REQ] Add option to fecth snapshot listings recursively

[parasiteoflife]

Is your feature request related to a problem? Please describe. When using remote repositories like Google Drive and rclone, the listing of the snapshots content is slow, since backrest doesn't recursively (--recursive) fetches the content of a snapshot the browsing is too slow.
There are some of us that would prefer to do a single long wait to several ones.

Describe the solution you'd like Add an option to enable recursively fetching the contents of snapshots, --recursive will do this.

Additional context