Add health checking API

After a Puppet Service can be health checked, then we will easy to make it manageable with K8S, HA Wechaty Proxy, etc.

See:

- [GRPC Health Checking Protocol](https://github.com/grpc/grpc/blob/master/doc/health-checking.md)
- [grpc_health_probe](https://github.com/grpc-ecosystem/grpc-health-probe)