Allow plugins to add data to the order API (Z#23179688) (#4822)

* Allow plugins to add data to the order API (Z#23179688)

* Update src/pretix/api/serializers/media.py

Co-authored-by: Richard Schreiber <schreiber@rami.io>

* Fix failing test

---------

Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/tests/api/test_reusable_media.py

@@ -203,7 +203,8 @@ def test_medium_detail(token_client, organizer, event, medium, giftcard, custome
             "canceled": False,
             "valid_from": None,
             "valid_until": None,
-            "blocked": None
+            "blocked": None,
+            "plugin_data": {},
         }
         assert resp.data["linked_giftcard"] == {
             "id": giftcard.pk,