Fixed #31007 -- Allowed specifying type of auto-created primary keys.

This also changes the default type of auto-created primary keys
for new apps and projects to BigAutoField.

tests/model_options/apps.py

@@ -0,0 +1,25 @@
+from django.apps import AppConfig
+
+
+class ModelDefaultPKConfig(AppConfig):
+    name = 'model_options'
+
+
+class ModelPKConfig(AppConfig):
+    name = 'model_options'
+    default_auto_field = 'django.db.models.SmallAutoField'
+
+
+class ModelPKNonAutoConfig(AppConfig):
+    name = 'model_options'
+    default_auto_field = 'django.db.models.TextField'
+
+
+class ModelPKNoneConfig(AppConfig):
+    name = 'model_options'
+    default_auto_field = None
+
+
+class ModelPKNonexistentConfig(AppConfig):
+    name = 'model_options'
+    default_auto_field = 'django.db.models.NonexistentAutoField'