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>.

apiextensions: enable CoreAPI options needed for admission

Admission webhooks need the client and the shared informers for kube resources. The comment is invalid and we have to enable the CoreAPI options.

This PR is important to run apiextensions-apiserver in a standalone integration test setup.