Use Preload function for external link provider

[martinlagler]

This part should be removed in the future, because there will be a ExternalLinkProvider with a preload function.
See: sulu/sulu#8044

        if ('external' === $redirectLink['provider']) {
            return $redirectLink['href'];
        }

https://github.com/sulu/SuluFormBundle/blob/2.5/Event/SendinblueListSubscriber.php#L162