Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

update admission webhook to accept client config

Fixes https://github.com/kubernetes/kubernetes/issues/53827

This plumbs a complete client through the plugin initializer for admission webhooks.  It achieves parity with our existing webhooks and provides flexibility if people want to do something special or different.  Easy things are easy, hard things are possible.  This does not change behavior for kube-apiserver.

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