Automatic merge from submit-queue (batch tested with PRs 49865, 53731, 54013, 54513, 51502). 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>.

 move webhook admission to generic apiserver

This builds on https://github.com/kubernetes/kubernetes/pull/54414.

It moves the admission webhook into the generic apiserver.  It also allows the injection of a server managed way to override the rest.Config used for a particular service.  This makes for an easier point to allow auto-configuration of the kube-apiserver to loopback to itself as a special case for kubernetes.default.svc.

@kubernetes/sig-api-machinery-pr-reviews 

```release-note
the generic admission webhook is now available in the generic apiserver
```