feat: implement memo relation server (#1618)

api/memo_relation.go

@@ -0,0 +1,19 @@
+package api
+
+type MemoRelationType string
+
+const (
+	MemoRelationReference  MemoRelationType = "REFERENCE"
+	MemoRelationAdditional MemoRelationType = "ADDITIONAL"
+)
+
+type MemoRelation struct {
+	MemoID        int
+	RelatedMemoID int
+	Type          MemoRelationType
+}
+
+type MemoRelationUpsert struct {
+	RelatedMemoID int
+	Type          MemoRelationType
+}