Add dropwizard health checks using gRPC health service

[msteinhoff]

gRPC provides an rpc service to check the health of an application and grpc-java already implements this:

https://github.com/grpc/grpc/blob/master/doc/health-checking.md
grpc/grpc-java@b7eea7c

The rough idea is to:

• extend the grpc server builder to optionally add such a health service
• add a new dropwizard health check that calls the health service

Still need to figure out

• how to integrate the health service with dropwizard, how to use ServingStatus and grpc-health can check specific service names
• what functionality the addon should provide
• what should be implemented by the application
• what defaults to use