private Optional<Component> buildTargetSelector(DeployState deployState, TreeConfigProducer<AnyConfigProducer> ancestor, Element searcherElement, ComponentId namespace) {
        Element targetSelectorElement = XML.getChild(searcherElement, "target-selector");
        if (targetSelectorElement == null)
            return Optional.empty();

        return Optional.of(new DomComponentBuilder(namespace).build(deployState, ancestor, targetSelectorElement));
    }
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