The generated code for gRPC-based clients has a lot of code that does not depend on the specific service: the generated client creates auth headers, implements the retry loop, adds the telemetry headers, etc.

We should refactor this code to google-cloud-gax-internal, where it can be reused and tested.

• Need to convert tonic::Status errors to our types
• Need to implement retry loop
• Need to implement timeouts per RPC
• Need to set idempotency from the generated code
• grpc::Client should use gax::Response
• Need to change the generated code to use this client
• Support user agent over gRPC #1881