Add support for Mailstep-based distribution of Special

We have a requirement to allow postal-fee-free delivery
of Special issues, which would go through the standard
print export channels.

We allow this by allowing modules to process MailStage
order items before they're sent to the Mailstage. Currently
we process any items with postal fee using the
"issue delivery" condition.

remp/respekt#320

src/Models/PaymentItem/GiftPaymentItem.php

@@ -56,4 +56,14 @@ public function data(): array
             'subscription_type_item_id' => $this->subscriptionTypeItemId,
         ];
     }
+
+    public function getSubscriptionTypeId(): int
+    {
+        return $this->subscriptionTypeId;
+    }
+
+    public function getSubscriptionTypeItemId(): ?int
+    {
+        return $this->subscriptionTypeItemId;
+    }
 }