Automatic merge from submit-queue
Add alpha command to kubectl
Also allow new commands to disable themselves by returning a nil value. This can be used to disable commands based on feature gates.
**What this PR does / why we need it**: Method of enabling alpha functionality in kubectl
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: ref #45922
**Special notes for your reviewer**: Part of a discussion in #45922 with @pwittrock
**Release note**:
```release-note
NONE
```
