Handle failure cases on startup gracefully to avoid causing cascading
errors and poor initialization in other components. Initial errors from
config load cause the initializer to pause and hold requests. Return
typed errors to better communicate failures to clients.
Add code to handle two specific cases - admin wants to bypass
initialization defaulting, and mirror pods (which want to bypass
initialization because the kubelet owns their lifecycle).
Automatic merge from submit-queue (batch tested with PRs 46239, 46627, 46346, 46388, 46524)
Dynamic webhook admission control plugin
Unit tests pass.
Needs plumbing:
* [ ] service resolver (depends on @wfender PR)
* [x] client cert (depends on ????)
* [ ] hook source (depends on @caesarxuchao PR)
Also at least one thing will need to be renamed after Chao's PR merges.
```release-note
Allow remote admission controllers to be dynamically added and removed by administrators. External admission controllers make an HTTP POST containing details of the requested action which the service can approve or reject.
```