Add healthz endpoints for health checking

[christophermaier]

This aligns better with conventions in Kubernetes, etc.

Extracted from #6023:

Add healthz endpoints to the REST API that mirror the standard health endpoints, but return 200 for healthy services and 500 otherwise. It would be beneficial to provide a health endpoint for the for the supervisor itself at /healthz, and a per-service endpoints at /services///healthz. The downside to this technique is that both the supervisor and Kubernetes perform checks periodically. If the supervisor checks health every 30 seconds, then it doesn't make sense for Kubernetes to check the REST API more often; but if the periods are offset the wrong way, there could potentially be up to a minute from a service problem -> supervisor health check -> Kubernetes readiness probe.

[stale]

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