Add rest endpoint annotations for health.proto

Please answer these questions before submitting your issue.

### What version of gRPC are you using?
Using gRPC 1.9.0

### What did you expect to see?
If it were possible to implement the health.proto service as REST interface too, it would minimize the duplication and generation of protofiles when using together with the repository of gRPC-gateway( https://github.com/grpc-ecosystem/grpc-gateway).