private Optional<FileReference> resolvePath(Optional<String> value) {
        return value.isEmpty() ? Optional.empty() : Optional.of(resolvePath(value.get()));
    }
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