Release v0.12.5

Added

• allow adding extra validation errors on an operation-basis using the new @extend_validation_errors decorator.
  You can find more information about that in the documentation.

Fixed

• use model._default_manager instead of model.objects.
• Don't generate error responses for OpenAPI callbacks.
• Make _should_add_http403_error_response check if permission is IsAuthenticated and AllowAny via type instead of isinstance
• Don't collect error codes from nested read_only fields