[FEATURE]: Export AbstractGenerator for custom generators in seed scripts

### Feature hasn't been suggested before.

- [x] I have verified this feature I'm about to request hasn't been suggested before.

### Describe the enhancement you want to request

I am using ulids for my ids and would like to have the ability to add custom generators to my seed refine to utilize my own id generator. However, the `AbstractGenerator` class is not exported for me to create my own generator