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

Start in generator/internal/scaffold_generator.cc at the referenced include-generation and List*() selection points. Trace how the scaffold generator currently runs without proto context, then determine how service definitions can be made available. Done means the quickstart skeleton emits the correct include and selects the first string-consuming List*() overload, falling back to "EDIT HERE" when none exists.

Written by the indexing model from the issue text.