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.
```