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
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