Automatic merge from submit-queue (batch tested with PRs 41541, 43710)

Admission plugin initializer for the generic API server.

**What this PR does / why we need it**: 
This PR implements a standard admission plugin initializer for the generic API server.
The initializer uses kubeconfig to populate external clients and informers. By default
in-cluster config is used.


**Special notes for your reviewer**:
https://github.com/kubernetes/community/blob/master/contributors/design-proposals/apiserver-build-in-admission-plugins.md

**Release note**: NONE

```release-note
```