Is your improvement request related to a feature? Please describe

Currently, the user can only takes/restores CSI snapshot when the target volume is attached.
We want to support taking/restoring CSI snapshots when the target volume is detached.
In order to do this, Longhorn needs to auto attach the target volume.

Additional context

We were thinking about doing auto attachment/detachment inside the Longhorn CSI plugin but at the end decided to wait for the refactoring #3715