Merge pull request kubernetes#649 from verb/pod-troubleshooting

Automatic merge from submit-queue.

Propose a feature to troubleshoot running pods

This feature allows troubleshooting of running pods by running a new "Debug Container" in the pod namespaces.

This proposal was originally opened and reviewed in kubernetes/kubernetes#35584.

This proposal needs LGTM by the following SIGs:
- [ ] SIG Node
- [ ] SIG CLI
- [ ] SIG Auth
- [x] API Reviewer

Work in Progress:
- [x] Prototype `kubectl attach` for debug containers
- [x] Talk to sig-api-machinery about `/debug` subresource semantics