Allow overriding snapshotclass per PVC

[JohnStrunk]

It should be possible to override the snapshotclass on a per-PVC basis by setting an annotation on the PVC.

In decreasing priority:

1. PVC annotation
2. class from the schedule
3. cluster default

[wjentner]

I'm currently running into this.
I have multiple storage classes, and I'd like to set up the scheduler for each of them.
Another solution for me would be to extend the claimSelector to also match for pvc.spec.volumeClassName

My current workaround is to use PVC labels, but this is error-prone as not all PVCs can be fully customized in all helm charts.