private FederationSearcherModel.TargetSpec readSource(Element source) {
            ComponentSpecification componentSpecification = XmlHelper.getIdRef(source);

            FederationOptions federationOptions =
                    readFederationOptions(XML.getChild(source,  FederationOptionsBuilder.federationOptionsElement));

            return new FederationSearcherModel.TargetSpec(componentSpecification, federationOptions);
        }
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