Add a secret key for ListSnapshot to SnapshotClass

[saad-ali]

Once CSI spec is update to add new secret field to ListSnapshotRequest (container-storage-interface/spec#370), we should:

1. Add a new key (something like csi.storage.k8s.io/list-snapshot-secret-name) to SnapshotClass.
2. Update kubernetes-csi docs to reflect what we support.

[fejta-bot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale.
Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Send feedback to sig-testing, kubernetes/test-infra and/or fejta.
/lifecycle stale

[xing-yang]

/remove-lifecycle stale

[fejta-bot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale.
Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Send feedback to sig-testing, kubernetes/test-infra and/or fejta.
/lifecycle stale

[xing-yang]

/remove-lifecycle stale

[xing-yang]

This is fixed by this PR: #252