Function oauth_callback_server 

pub async fn oauth_callback_server(
    expected_state: String,
    tx: UnboundedSender<Result<String, Error>>,
) -> (impl Future<Output = Result<(), Error>>, u16)

Binds a loopback listener for an OAuth authorization response that must carry expected_state, and sends the authorization code (or the error the authorization server returned) to tx.