Add GetVolumeSnapshotClassFromVolumePolicy helper and extend
GetVolumeSnapshotClass with a policySnapshotClass parameter. The new
tier sits between PVC annotation and backup annotation in the priority
chain: PVC annotation > volume policy > backup annotation > VSC label.
Ref: #8807
Signed-off-by: Shubham Pampattiwar <spampatt@redhat.com>