[CSI Plugin] Make VolumeSnapshotClass label optional

[rnarenpujari]

Describe the problem/challenge you have
As a prerequisite to taking a backup with CSI snapshots, we are required to manually label a volume snapshot class with velero.io/csi-volumesnapshot-class: "true" for every every csi driver being used. This seems to be needed in order to disambiguate with class to use in case there are multiple for the same driver.

Describe the solution you'd like
It would be nice to have zero configuration since in the common scenario there is probably only a single snapshot class per driver. Even if there are multiple, we could have some arbitrary selection criteria (random, oldest resource, sorted on name, etc.) by default and then allow the user to use the label for more precise behavior.

Anything else you would like to add:
Relevant code

Environment:

• Velero version (use velero version): 1.10.0
• Kubernetes version (use kubectl version):
• Kubernetes installer & version:
• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):

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[draghuram]

You may be interested in #5774. In general, it is better to pick volume snapshot class in a deterministic manner.

[reasonerjt]

Likely a dup of #5750

[reasonerjt]

This is not a dup b/c the requirement looks for zero-configuration, however, user will need to create a vsClass anyway.

Per discussion with @rnarenpujari , this is a good-to-have but not very high priority.

[reasonerjt]

It should be do-able that we fall back to the vsClass as long as there is no ambiguity