public List<SearchChainInvocationSpec> allProviderSources() {
        List<SearchChainInvocationSpec> allProviderSources = new ArrayList<>();
        for (ComponentAdaptor<SearchChainInvocationSpec> component : providerSources.allComponents()) {
            allProviderSources.add(component.model);
        }
        return allProviderSources;
    }
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