mgr/telemetry: add preview-device and preview-all commands
`ceph telemetry show` will show a sample cluster report if the user is
opted-in to telemetry. The report will be compiled of the collections
the user is opted-in to. To preview a report compiled of the most recent
collection available, use `ceph telemetry preview`.
The device channel is not included in the cluster report, since it's
being sent to a different endpoint, thus we use
`ceph telemetry show-device` in case the user is opted-in to telemetry
and the device channel is enabled. If not, it can also be previewed with
`ceph telemetry preview-device`.
If telemetry is on, and device channel is enabled, both reports can be
reviewed with `ceph telemetry show-all`, otherwise use
`ceph telemetry preview-all`.