Consider re-exporting *-gax Paginator traits

[suzmue]

https://googleapis.github.io/google-cloud-rust/setting_up_your_development_environment.html and https://googleapis.github.io/google-cloud-rust/setting_up_rust_on_cloud_shell.html both use google_cloud_gax directly.

We should consider if we would rather re-export the necessary traits from the client libraries, as is done for gax::Result and lro::Poller.

See https://github.com/googleapis/google-cloud-rust/issues/2108#issuecomment-2874054156 for context, and also #903 / #919