More accessible auth error and avoid multiple types

[thomasa88]

Is your feature request related to a problem? Please describe.
When the refresh token no longer works for the client, I want to send the user back to the authorization flow.

My code currently looks something like this:

        let client_service = ClientInfo::default();
        let mut mcp_client_res = client_service.serve(transport).await;

        if let Err(rmcp::service::ClientInitializeError::TransportError {
            error: dyn_transport_err,
            context: _,
        }) = &mcp_client_res
        {
            debug!("Transport error: {dyn_transport_err:#?}");
            // Try again if it was an authorization error
            if Self::is_auth_required_error(dyn_transport_err) {
                // new auth
             }
        }


    fn is_auth_required_error(dyn_transport_err: &rmcp::transport::DynamicTransportError) -> bool {
        let http_error = dyn_transport_err
            .error
            .downcast_ref::<rmcp::transport::streamable_http_client::StreamableHttpError<
            reqwest::Error,
        >>();
        matches!(
            http_error,
            Some(
                rmcp::transport::streamable_http_client::StreamableHttpError::Auth(
                    rmcp::transport::AuthError::AuthorizationRequired,
                ) | rmcp::transport::streamable_http_client::StreamableHttpError::AuthRequired(
                    rmcp::transport::streamable_http_client::AuthRequiredError { .. }
                )
            )
        )
    }

I have to first downcast the error and then match on two different authorization errors.

Describe the solution you'd like

Avoid the downcast if possible. Return one error for authorization required instead of two.

Describe alternatives you've considered

Additional context