Expose grpc_health_v1 descriptor set in tonic-health for use with the reflection service

[byblakeorriver]

It would be nice to expose the grpc_health_v1 descriptor set in the tonic-health container so that it can be accessed for registering with the reflection service in tonic-reflection.

In order to use tools like grpcui and grpcurl the grpc server has to expose the reflection service. Thus, if I wanted to hit the health endpoints with those tools, I would have to register the descriptor set of the health service in the reflection service. Currently, I have to keep a copy of the health.proto file locally and build the descriptor set with prost. This could be alleviated by building and exposing the descriptor set in the tonic-health crate.

If this is something that would be acceptable, I could open a PR with the changes.