ws-manager: add support for deleting snapshotVolumes (to clean up old backups) on workspace delete

[sagor999]

• Support for deleting old volume snapshot when new one is created for workspace
• Support for restoring volume snapshot from the cloud when it doesn't exist yet in the cluster.

[sagor999]

We need to solve following cases:

• New workspace volume snapshot is created. We need to delete old volume snapshot for that workspace now.
• Workspace is deleted. We need to delete any volume snapshots that might be remaining.
• Open workspace in the cluster where volume snapshot doesn't exist yet. We need to restore volume snapshot.