Add service name in generated gRPC code

When doing a [gRPC health check](https://github.com/grpc/grpc/blob/master/src/proto/grpc/health/v1/health.proto), we can specify the service name to check. However, there's no way to extract that from the generated code on the client without creating a stub server, e.g.

```dart
doHealthCheck() {
final client = HealthClient(channel, options: defaultCallOptions);
final healthResponse = await client.check(
HealthCheckRequest()
..service = StubChessService().$name
..freeze());
}

class StubChessService extends ChessService {
...
}
```

It would be helpful to have a static serviceName on the generated client to avoid magic strings or creating stubs.