Automatic merge from submit-queue (batch tested with PRs 51471, 50561, 50435, 51473, 51436)

Allow zsh completion to be autoloaded by compinit

**What this PR does / why we need it**:
Allows the kubectl zsh autocompletion to be auto loaded by compinit. Had to move the the boilerplate down into the specific shell functions as the compdef needs to be the first line in the definition file.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #50560

**Special notes for your reviewer**:

**Release note**:

```release-note
kubectl zsh autocompletion will work with compinit
```