Support gRPC health check endpoint by default

[TannerYoung]

Mu should support gRPC health checks with limited configuration (if not by default) out of the box. See https://github.com/grpc/grpc/blob/master/doc/health-checking.md for details.

This would allow dependencies to wait for the service to become healthy before sending requests to it. For example https://github.com/grpc-ecosystem/grpc-health-probe/

This could be done with a default GrpcConfig implementation to be added as a parameter to configList in GrpcServer.netty[IO](port, myService :: healthService :: Nil).

[ememmr]

A first version of health check service: #630
Pending:

• Create new issue: "unary service part should support not only protobuf but also avro" (HealthCheck endpoint and service #630 (comment))
• Create healthcheck documentation (?)