add example of validating admission webhook

[pohly]

In Kubernetes 1.31, custom configuration parameters will get embedded inside ResourceClaim and ResourceClaimTemplate. They are clearly marked as belonging to a certain DRA driver, so an admission webhook provided together with that driver can check those configuration parameters early instead much later in the NodePrepareResources call.

While writing a webhook is documented elsewhere, having an example that is specific to these types is still valuable.

[pohly]

/help

[k8s-ci-robot]

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[nojnhuh]

/assign