Automatic merge from submit-queue (batch tested with PRs 45272, 45115)
initial types for TPRs
This pull starts creating the types described by https://github.com/kubernetes/community/blob/master/contributors/design-proposals/thirdpartyresources.md . In the initial pull different names were suggested. I've started this pull with `CustomResource.apiextensions.k8s.io`.
The structure begins as a separate API server to facilitate rapid prototyping and experimentation, but the end result will be added to the end of the `kube-apiserver` chain as described in https://github.com/kubernetes/community/blob/master/sig-api-machinery/api-extensions-position-statement.md .
Because it is separate to start (not included in any default server), I don't think we need a perfect name, but I'd like to be close.
@kubernetes/sig-api-machinery-misc @enisoc @smarterclayton @erictune
df8551acd2