Add healthcheck endpoint/micrometer

[iyourshaw]

LOWER PRIORITY, not a core requirement

Add a new healthcheck GET endpoint. This would be useful for Kubernetes clusters to have to know when to automatically restart malfunctioning pods.

• The endpoint may returns HTTP status 200 if all topologies are running, or 500 http status if any topologies are in an error state. Return a JSON body in case of error, indicating which topology is down, or other details to be logged.
• Consider using Micrometer (implemented with Spring Boot Actuator) to implement the health check endpoint, and provide any other monitoring metrics needed.
• Leave the existing /health endpoints as is, returning human-readable detailed diagnostic information.

CI Docker build:

Consider using a health check in the Github Docker CI test to verify the container is running, in addition to that it just builds.