Feature request: custom complex types

[gsakkis]

Currently pydantic-settings determines whether a field is "complex" (nested) or not based on its annotation as follows:

def _annotation_is_complex_inner(annotation: type[Any] | None) -> bool:
    if lenient_issubclass(annotation, (str, bytes)):
        return False

    return lenient_issubclass(annotation, (BaseModel, Mapping, Sequence, tuple, set, frozenset, deque)) or is_dataclass(
        annotation
    )

It would be convenient if one could register other settings-like types as complex (either per model or globally) so that nested env variable explosion works. Case in point, a GenerationConfig from Hugging Face transformers.

Draft API:

class TransfomerModelSettings(BaseSettings):
    model_config = SettingsConfigDict(
        env_prefix="model__", complex_annotations=(GenerationConfig,)
    )

    name_or_path: str
    generation_settings: GenerationConfig = GenerationConfig()

Then one could pass via env vars any of the dozens GenerationConfig fields:

export model__name_or_path=foo 
export model__max_new_tokens=123
...