api2/tape/restore: add optional snapshots to 'restore'

api2/tape/restore: add optional snapshots to 'restore'

this makes it possible to only restore some snapshots from a tape media-set
instead of the whole. If the user selects only a small part, this will
probably be faster (and definitely uses less space on the target
datastores).

the user has to provide a list of snapshots to restore in the form of
'store:type/group/id'
e.g. 'mystore:ct/100/2021-01-01T00:00:00Z'

we achieve this by first restoring the index to a temp dir, retrieving
a list of chunks, and using the catalog, we generate a list of
media/files that we need to (partially) restore.

finally, we copy the snapshots to the correct dir in the datastore,
and clean up the temp dir

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>