proper support of __get_pydantic_core_schema__

### Summary

I'd like to dynamically add non-public methods to my Enums

### What is the feature request for?

The core library

### The Problem

I'm mixing up betterproto.Enum with pydantic classes for easier conversion between my database objects and protobuf messages. To use pydantic, I have to add validator based on [this instruction](https://docs.pydantic.dev/2.0/usage/types/custom/#handling-third-party-types). Thus, I decided to dynamically add a class method to a generated betterproto.Enum. However, betterproto.enum does not allow it by [this](https://github.com/danielgtaylor/python-betterproto/blob/master/src/betterproto/enum.py#L109)

Are there any other parts of the code affected by adding a custom function `__get_pydantic_core_schema__` to my generated Enum?

### The Ideal Solution

I'd like `__set_attr__` to accept new attributes if the new attributes are private (i.e.,starting with `__`) I can contribute if this direction is acceptable.

### The Current Solution

If no other solutions, I will fork this repo and just remove the constraint. I don't like to fork for a hack.