Exposing `_get_fetch_fields`

Is your feature request related to a problem? Please describe.
I am running a project with FastAPI and Pydantic, when I find that I often want to implement custom from_queryset, from_tortoise_orm, or from_queryset_single methods. I don't want to change much of the original behavior, I just want to add some extra prefetching (which is overridden if I try to prefetch before calling those methods), or sometimes I would like to inject a constant value for all objects in a queryset.

Describe the solution you'd like
I think exposing _get_fetch_fields as a classmethod would really help ease overriding the aforementioned methods. Something as simple as:

class PydanticModel(BaseModel):
    .....

    @classmethod
    def get_default_fetch_fields(cls) -> List[str]:
        return _get_fetch_fields(cls, getattr(cls.__config__, "orig_model"))

    ....

Describe alternatives you've considered
I've tried annotating a queryset with a constant value before serializing it with from_queryset, but the annotate method only allows expressions and terms. I've considered importing _get_fetch_fields and using it, but it's a private helper and I'm worried it could cause undocumented breaking changes in the future. I've also considered looping over the queryset, adding in a constant value, and then running a from_tortoise_orm to serialize the objects, but I think that's much more inefficient than a prefetch on a queryset (compared to fetch_related on every iteration, however, I'm not entirely sure).

Additional context
This is just a suggestion, I am very open to criticism and alternatives/ideas I have not thought of.

Also, thank you for an awesome ORM! 🎉

Start by locating the private _get_fetch_fields helper and the from_queryset, from_tortoise_orm, and from_queryset_single methods mentioned in the issue. Review how their default fetch fields are determined, then check whether a classmethod exposing that behavior can be added without changing existing serialization; done means custom methods can reuse it and the relevant tests pass.