add invoice pdf

lib/domain/constants.dart

@@ -91,3 +91,9 @@ const shipment_pickuped_status = "pickuped";
 const shipment_packed_status = "packed";
 const shipment_shipped_status = "shipped";
 const shipment_delivered_status = "delivered";
+
+// invoice status
+const invoice_issued_status = "issued";
+const invoice_saved_status = "saved";
+const invoice_cancel_status = "canceled";
+const invoice_paid_status = "paid";