private ComponentSpecification providerSpecificationForSource(Properties queryProperties) {
        String spec = queryProperties.getString("source." + sourceId().stringValue() + ".provider");
        return ComponentSpecification.fromString(spec);
    }
Looks up source.(sourceId).provider in the query properties.
@return null if the default provider should be used
container-search/src/main/java/com/yahoo/search/federation/sourceref/SourcesTarget.java