private FederationOptions readFederationOptions(Element federationOptionsElement) {
            if (federationOptionsElement == null) {
                return new FederationOptions();
            } else {
                return new FederationOptionsBuilder(federationOptionsElement).build();
            }
        }
Builds a federation searcher config producer from an element.

@author Tony Vaagenes
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/chains/search/DomFederationSearcherBuilder.java