public File value() {
        return value;
    }
Represents a 'url' in a {@link ConfigInstance}, which will be downloaded
and made available as a {@link File}. Stored in the config builder as a
{@link UrlReference} to identify fields of this type for special handling
in the ConfigPayloadApplier.

@author lesters
config-lib/src/main/java/com/yahoo/config/UrlNode.java