Automatic merge from submit-queue
Support exec/attach/portforward in `kubectl proxy`
Use the UpgradeAwareProxy shared code in kubectl proxy. Provide a separate transport for those requests that does not have HTTP/2 enabled. Refactor the code to be a bit cleaner in places and to better separate changes.
Fixes#32026
```release-note
`kubectl proxy` will now correctly handle the `exec`, `attach`, and `portforward` commands. You must pass `--disable-filter` to the command in order to allow these endpoints.
```
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