Automatic merge from submit-queue (batch tested with PRs 45996, 46121, 45707, 46011, 45564)
add "admission" API group
This commit is an initial pass at providing an admission API group.
The API group is required by the webhook admission controller being
developed as part of https://github.com/kubernetes/community/pull/132
and could be used more as that proposal comes to fruition.
**Note:** This PR was created by following the [Adding an API Group](https://github.com/kubernetes/community/blob/master/contributors/devel/adding-an-APIGroup.md) documentation.
cc @smarterclayton