Provide a gRPC server health check

**The problem**

Monitor the health of a gRPC server app.

**The solution**

Provide a gRPC server `HealthEndpoint`. My blog post on this matter may be useful.
https://blog.asarkar.com/technical/grpc-kubernetes-spring/

**Alternatives considered**

Implement myself.

**Additional context**

Also note that starting with version 2.3.0.RELEASE, Spring Boot provides liveness and readiness information under Actuator health endpoint. See [Kubernetes Probes](https://docs.spring.io/spring-boot/docs/current/reference/html/production-ready-features.html#production-ready-kubernetes-probes) for details.