Fix volumesnapshot webhook-example RBAC rules

[astraw99]

What type of PR is this? /kind bug

What this PR does / why we need it: Adds RBAC volumesnapshots and volumesnapshotcontents to the validating webhook example, so that it can be deployed with proper access to the resources list/watch.

Does this PR introduce a user-facing change?:

NONE

[RaunakShah]

Can you update the description with what was failing and how this change fixes the issue?

[k8s-ci-robot]

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