optional callable on `new-class` validator

We need to an an optional callable to the `new-class` schema/validator to allow `_init_private_attributes` to be called on models.

See [here](https://github.com/pydantic/pydantic/blob/169db6e0be2954e7e4160353e0b2a2ba19873cd2/pydantic/main.py#L136):

```py
__pydantic_self__._init_private_attributes()
```