Velero: [v0.10] update `ark backup describe` for the changes to volume snapshot info storage

[skriss]

With #922, information about volume snapshots is no longer stored as part of the Backup CRD. The ark backup describe command used to display this information directly from the status. It needs to be updated to work with the new design. My proposal:

• [ ] add 2 new fields to Backup's status, VolumeSnapshotsAttempted and VolumeSnapshotsSucceeded
• [ ] by default, ark backup describe will only display this summary information (e.g. 2 of 3 volume snapshots completed successfully)
• [ ] change the current --volume-details flag on ark backup describe (used to show details of restic snapshots) to a generic --details flag
• [ ] add a new DownloadRequest target type, for the <backup>-volumesnapshots.json.gz file in object storage
• [ ] if the --details flag on ark backup describe is specified, download the volumesnapshots file from obj storage (similarly to how restore results are downloaded) and display the detailed info

@nrb @wwitzel3 @carlisia @ncdc WDYT?

[ncdc]

It all seems reasonable to me. I wonder if people will notice and understand the --details flag?

[skriss]

Currently the output of the describer if --volume-details is not specified has a note that it can be specified to get more info -- I'd probably do the same here so folks know it's an option

[carlisia]

Yes, this makes sense. 👍

[nrb]

@skriss Yep, I'd agree with having the note so it's more easily discoverable.

For your first point, do we want to add a Prometheus metric to that, based on schedules like the rest?

The rest seems reasonable.