Add reference contracts to extractor metadata

internal/framework/pipeline/walking_skeleton_test.go

@@ -252,6 +252,10 @@ func (extractor walkingSkeletonExtractor) SchemaVersion() string {
 	return "v1"
 }
 
+func (extractor walkingSkeletonExtractor) ReferenceSlots() []contracts.ReferenceSlot {
+	return nil
+}
+
 func (extractor walkingSkeletonExtractor) Validators() []contracts.Validator {
 	return nil
 }