Consider using a repeated string field as the paginated item by default

Originally from @dbolduc in https://github.com/googleapis/google-cloud-rust/issues/4687#issuecomment-3915935492:

> FWIW, in C++ we said if there was only a repeated string field, use that as the paginated item:
>
> https://github.com/googleapis/google-cloud-cpp/blob/6a4b2fb26e51a9af7c8521e596806b9c563a9ee9/generator/internal/pagination.cc#L87-L93
>
> That strategy has a failure mode. If we pick some string field at first, then a repeated message field is later added to the API, the paginated type switches, breaking the API. Seems super unlikely though.
>
> Might be worth checking if there are other APIs we can fix, or if Pub/Sub is the only one at issue.