Use first client with a `List*()` operation for quickstart skeleton

The scaffold generator runs without any context of the protos in use. We could move the generation of the quickstart skeleton to have access to the service definition. With that information we could generate a correct include file here:

https://github.com/googleapis/google-cloud-cpp/blob/43cd3d9a65df32310bca8d8878ee322d73493a03/generator/internal/scaffold_generator.cc#L938

and maybe pick the first `List*()` overload consuming a string (with some fallback to "EDIT HERE" if there is none):

https://github.com/googleapis/google-cloud-cpp/blob/43cd3d9a65df32310bca8d8878ee322d73493a03/generator/internal/scaffold_generator.cc#L953