close #4763: client: add command to forget backup group

close #4763: client: add command to forget backup group

Add the command `proxmox-backup-client group forget <group>` so
that we can forget (delete) whole groups with all the containing
snapshots.
To avoid printing full datastore paths (which are in the error messages)
we filter out the most common one (group not found) and rephrase it.

Signed-off-by: Gabriel Goller <g.goller@proxmox.com>
[WB: rebased & sorted import statements in client's main.rs]
[WB: replace extract_repository_from_value with
     remove_repository_from_value since the parameter is rejected on
     the remote side]
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>