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
```