ClusterID for ClusterSet identification (kubernetes#2150)

* Move clusterID doc to KEP draft

* Add reviewers/approvers to yaml

* Remove lingering references to signing/validation for future iteration

* Add encouragement to migrate to clusterID

Explain our position in regards to existing cluster ID
implementations directly in the Summary and Motivation sections

* Adding caveat on name

* Clean up well known claim language & add scenarios

* Restrict location of additional claims further

* Add request for DNS examples

* Specify multi-clusterset clusters as a nongoal

* Add some notes from implementation discussions

* Add placeholder for DNS

* Strengthen immutability encouragement, formatting, link ref

* Formatting, add clarity on well known claims

* Add unresolved point on id uniqueness in previously isolated clusters

* Add ClusterClaim content size and explanation on loose schema

* fix typo

* reorg/add words to user stories

* Add notes from call re: beta graduation and aliases

* add user stories to goals

* Update toc

* update toc links for user stories

* fix typo

* update yaml

* add prr approver to yaml

* fix spelling

* copy over toc from toc test

Author: lauralorenz