feat: add possibility to return primary keys instead of nested models, set by model incl. nesting

[collerek]

flatten_fields

flatten_fields(columns: Union[list, str, set, tuple, dict, FieldAccessor]) -> QuerySet

With flatten_fields() you can render selected related models as their primary
key value on model_dump() instead of the default nested dict. This is useful
when your API clients expect {"manufacturer": 1} rather than
{"manufacturer": {"id": 1, "name": "Toyota", ...}}.

Accepts the same input forms as fields() / exclude_fields() (string, list,
set, tuple, dict-with-Ellipsis) plus FieldAccessor / list of accessors.
Works across foreign keys, many-to-many, and reverse relations.

all_cars = await (
    Car.objects
    .select_related('manufacturer')
    .flatten_fields('manufacturer')
    .all()
)
assert all_cars[0].model_dump() == {
    'id': 1,
    'name': 'Corolla',
    'manufacturer': 1,  # flattened from nested dict to pk value
}

[codspeed-hq]

Merging this PR will degrade performance by 18.11%

❌ 1 (👁 1) regressed benchmark
✅ 83 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
👁	WallTime	test_making_and_inserting_models_in_bulk[10]	8.9 ms	10.9 ms	-18.11%

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_{Comparing feature/flatten-fields (80187d8) with master (672db74)}