Add reference contracts to extractor metadata

internal/framework/pipeline/registry_integration_test.go

@@ -149,6 +149,10 @@ func (extractor integrationExtractor) SchemaVersion() string {
 	return "v1"
 }
 
+func (extractor integrationExtractor) ReferenceSlots() []contracts.ReferenceSlot {
+	return nil
+}
+
 func (extractor integrationExtractor) Validators() []contracts.Validator {
 	return extractor.validators
 }