Automatic merge from submit-queue (batch tested with PRs 49619, 49598, 47267, 49597, 49638)

Flag support in kubectl plugins

Adds support to flags in `kubectl` plugins. Flags are declared in the plugin descriptor and are passed to plugins through env vars, similar to global flags (which already works).

Fixes https://github.com/kubernetes/kubernetes/issues/49122

**Release note**:

```release-note
Added flag support to kubectl plugins
```
PTAL @monopole @kubernetes/sig-cli-pr-reviews