Automatic merge from submit-queue

allow combining API servers

Builds on https://github.com/kubernetes/kubernetes/pull/42886 (already lgtm'ed)

We need to be able to chain multiple API servers together so that a fallthrough case from to another results in delegated handling without double wrapping. We also need to be able to combine shared lists like healthz and poststarthooks so that a single API server start will run all the poststarthooks and present a unified view of health.  This creates an interface and methods to provide that wiring.

@kubernetes/sig-api-machinery-misc @ncdc