Set default value for is_composite_pk in admin config

Added an `is_composite_pk` attribute with a default value of `False` to the admin configuration. This ensures clarity in object definition and supports better configurability for composite primary key scenarios.

core/admin.py

@@ -384,6 +384,7 @@ class ConstanceConfig:
         verbose_name_plural = _("config")
         abstract = False
         swapped = False
+        is_composite_pk = False
 
         def get_ordered_objects(self):
             return False