Add setter to auth Config

[yoshidan]

Before

 let config = google_cloud_auth::project::Config {
      audience: Some(crate::apiv1::conn_pool::AUDIENCE),
      scopes: Some(&crate::apiv1::conn_pool::SCOPES),
      sub: None,
      ..Default::default()
  }

After

   let config = google_cloud_auth::project::Config::default()
            .with_audience(crate::apiv1::conn_pool::AUDIENCE)
            .with_scopes(&crate::apiv1::conn_pool::SCOPES);