MCS: Add Headless support and graduation criteria (kubernetes#1810)

* add test plan, upgrade, version skew, graduation

Other clarification based on PR comments

* first pass at headless service support

* move cluster state to SI.Status

* address PR comments

- add notes on headless services
- add danwinship as a reviewer

* fix service port definition

* git address feedback from API review

add deads2k and liggitt as reviewers

* Change to implementable

- Add clarification around handling of missing namespaces
- Add cluster ID exploration to graduation requirements
- Clarify cluster name change behavior expectations
- Refine conditions

* Add local definition of ServicePort

- include only in-scope fields
- recommendations around endpoint TTL
- suggest preventing non-admin ServiceImport writes

Author: JeremyOT