New comments section in the requests/responses

frontend/pshared/test/payment/request_dto_format_test.dart

@@ -57,6 +57,7 @@ void main() {
           ),
           amount: MoneyDTO(amount: '10', currency: 'USD'),
           settlementMode: 'fix_received',
+          comment: 'invoice-7',
         ),
       );
 
@@ -70,6 +71,7 @@ void main() {
       final intent = json['intent'] as Map<String, dynamic>;
       expect(intent['kind'], equals('payout'));
       expect(intent['settlement_mode'], equals('fix_received'));
+      expect(intent['comment'], equals('invoice-7'));
       expect(intent.containsKey('settlement_currency'), isFalse);
 
       final source = intent['source'] as Map<String, dynamic>;