this is a two stage refactor when done there will be no init block in admission plugins.
Instead all plugins expose Register function which accept admission.Plugins instance.
The registration to global plugin registry happens inside Register func.
Automatic merge from submit-queue (batch tested with PRs 44440, 44038, 44302, 44316, 43876)
Extend the gc admission plugin to check ownerReference.blockOwnerDeletion
#Extend the gc admission plugin to prevent user who doesn't have delete permission of the *owner* from changing blockOwnerDeletion field of existing ownerReferences, or adding ownerReference with blockOwnerDeletion=true.
The plugin need a RESTMapper to translate ownerRef.Kind to Resource. It should be using a dynamic one. However, as discussed in https://github.com/kubernetes/kubernetes/pull/42615, such a RESTMapper will be built after watchable discovery API is implemented, so in this PR the plugin is using the `api.Registry.RESTMapper()`, which is also [used](https://github.com/kubernetes/kubernetes/blob/master/cmd/kube-controller-manager/app/core.go#L165-L166) by the garbage collector currently.
```release-note
Extending the gc admission plugin so that a user who doesn't have delete permission of the *owner* cannot modify blockOwnerDeletion field of existing ownerReferences, or add new ownerReference with blockOwnerDeletion=true
```
cc @lavalamp
Automatic merge from submit-queue
Non global admission plugin registry
For testing the global state is a problem. This PR turns the actual registry into a struct that must be instantiated. For the beginning, we do this in `pkg/kubeapiserver/admission`. In some follow-up (where we hunt down all globals some day), we will move this into the genericapiserver.
The exported or public functions requires a doc comment to pass golint.
This commit has changes of conversion generated code. The actual doc
changes are added into a separate commit for a clean review.
