Is your feature request related to a problem? Please describe.
Currently, Longhorn can do crash-consistent snapshot/backup. But for the applications like databases, quiescing is needed to create an application-consistent snapshot, so the application will make sure all the data in the memory has been written to the disk before creating the snapshot.

Describe the solution you'd like
Provide a way for the users to quiesce the workload before taking the snapshot/backup.

Describe alternatives you've considered
Users can also script the backup using CSI snapshotter and run quiescing/unquescing before taking the snapshot. But it won't work with the recurring snapshot/backup which is scheduled by Longhorn.

Additional context
One application is normally composed of multiple workloads, so we need to consider volume group scenario as well.