Merge pull request #58697 from liggitt/aggregator-e2e-fix

Automatic merge from submit-queue (batch tested with PRs 58697, 58658, 58676, 58674). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Wait for healthy extension server before registering APIService, handle ServiceUnavailable errors

fixes #58642
followup to #58070

* Because a registered APIService appears in discovery immediately, we should wait until the backing deployment is healthy before exposing it
* In e2e hasRemainingContent(), add ServiceUnavailable to the types of errors we tolerate when looking for remaining content.
* In proxy handler, return a ServiceUnavailable error if the referenced service cannot be resolved
```release-note
NONE
```

Kubernetes-commit: 8d62044e73320b2385f3ac3cc1b5e34598ed12a4

Author: k8s-publishing-bot