Bug 15516: (follow-up) Add hold_group_id to hold REST API definition

Brought to light by accessing the following page with the feature enabled:
http://localhost:8081/cgi-bin/koha/members/holdshistory.pl?borrowernumber=51

Also by running the following tests file before this patch:
t/db_dependent/api/v1/holds.t

Signed-off-by: Pedro Amorim <pedro.amorim@openfifth.co.uk>
Signed-off-by: Andrew Fuerste Henry <andrew@bywatersolutions.com>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>

api/v1/swagger/definitions/hold.yaml

@@ -107,6 +107,10 @@ properties:
       - "null"
     description: If record level hold, the optional itemtype of the item the patron is
       requesting
+  hold_group_id:
+    type:
+      - integer
+      - "null"
   item_level:
     type: boolean
     description: If the hold is placed at item level