Merge pull request #54889 from lavalamp/wh-api

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>.

Fix webhook API to also support URLs

ref: kubernetes/enhancements#492

```release-note
The dynamic admission webhook now supports a URL in addition to a service reference, to accommodate out-of-cluster webhooks.
```

Kubernetes-commit: e93819049db49694718bc9c96e67050d366c6f63