private UrlReference resolveUrl(String url) {
        if ( ! isClientside()) return new UrlReference(url);
        File file = urlDownloader.waitFor(new UrlReference(url), Duration.ofMinutes(60));
        return new UrlReference(file.getAbsolutePath());
    }
This may run on both the config server and subscribing nodes.
We only have a urlDownloader set up when client side.
config/src/main/java/com/yahoo/vespa/config/ConfigPayloadApplier.java