Adds a `base64decode` function to templates in `kubectl` so that
it's possible to extract secret data in plaintext instead of
base64 without requiring a separate executable.
Sample usage:
```sh
kubectl get secret SECRET -o go-template='{{ .data.KEY | base64decode }}'
```
This fixes#38779.
This allows us to avoid case in which printers.GetStandardPrinter
returns nil for both printer and err removing any potential panics that
may arise throughout kubectl commands.
Please see #38779 and #38112 for complete context.
Add comment explaining adding handlers to printers.HumanReadablePrinter
also remove an unnecessary conversion of printers.HumanReadablePrinter
to printers.ResourcePrinter.
