chore: add parent field to memo

store/memo.go

@@ -42,6 +42,9 @@ type Memo struct {
 	Visibility Visibility
 
 	// Composed fields
+	// For those comment memos, the parent ID is the memo ID of the memo being commented.
+	// If the parent ID is nil, then this memo is not a comment.
+	ParentID       *int32
 	Pinned         bool
 	ResourceIDList []int32
 	RelationList   []*MemoRelation