Automatic merge from submit-queue (batch tested with PRs 41121, 40048, 40502, 41136, 40759)

add k8s.io/sample-apiserver to demonstrate how to build an aggregated API server

builds on https://github.com/kubernetes/kubernetes/pull/41093

This creates a sample API server is a separate staging repo to guarantee no cheating with `k8s.io/kubernetes` dependencies.  The sample is run during integration tests (simple tests on it so far) to ensure that it continues to run.

@sttts @kubernetes/sig-api-machinery-misc ptal
@pwittrock @pmorie @kris-nova an aggregated API server example that will stay up to date.