Function provider_redirect 

async fn provider_redirect(
    services: &Services,
    client: IpAddr,
    context: Flow<'_>,
    idp_id: &str,
) -> Result<Redirect>

Send the browser to the provider chosen on the login page.

The pending request is bound to that provider only once its URL is built, and the binding is final, so the request cannot also complete with a local password or another provider.