Add is_default field to /admin/libraries response

it's not super clear currently in the admin UI that collections
defined for default library are applied to all other libraries.

By passing this extra field we can then display some extra info
on the admin UI side.

api/admin/controller/library_settings.py

@@ -83,6 +83,7 @@ def process_get(self) -> Response:
                     name=library.name,
                     short_name=library.short_name,
                     settings=settings,
+                    is_default=library.is_default,
                 )
             ]
         return Response(