Automatic merge from submit-queue (batch tested with PRs 50911, 50839)
Fix admission plugin registration
**What this PR does / why we need it**:
The current registration of admission plug-ins happen too late so the
command line help is not yielding correct/useful information to users.
This is already causing confusion as mentioned in some issues.
This patch fixes it by moving plugins.go down to options package because
"plugins" in this context are themselves options for users. Registration
of plugins is not an expensive operation and it is already done in most
execution paths.
**Which issue this PR fixes**
fixes#50833
**Special notes for your reviewer**:
There might be other thoughts on how to better align this with other
efforts. This PR is leaving them to future work. For example:
in future, we may want to revisit these plugins and migrate them to the
shared apiserver repo when appropriate.
**Release note**:
I'm not sure if a release note is needed for this. Maybe there is a need.
```release-note
```
0ade03bc0f