✨(backend) support add-ons authentication in external viewset

Integrate the add-ons authentication backend into the externalviewset.

This allows third-party integrations (e.g. calendar add-ins) to
obtain a JWT for a user and use it to call the API (e.g. create a
room) via a Bearer authorization header.

src/backend/core/external_api/viewsets.py

@@ -175,6 +175,7 @@ class RoomViewSet(
 
     authentication_classes = [
         authentication.ApplicationJWTAuthentication,
+        authentication.AddonsJWTAuthentication,
         ResourceServerAuthentication,
     ]
     permission_classes = [