feat: Let constructor fixtures return narwhals object directly

[FBruzzesi]

Description

Alright, at this point I briefly mention this branch to all of you @MarcoGorelli @dangotbanned @camriddell so I might as well open the PR.

I think it is promising, but I might need some help for the typing, here is a repro for the issue I am facing:

from __future__ import annotations

from typing import TYPE_CHECKING, reveal_type

import narwhals as nw

if TYPE_CHECKING:
    from narwhals.testing.typing import FrameConstructor


def test_typing(nw_frame: FrameConstructor) -> None:
    data = {"a": [-1, 2, 3], "b": [1, 1, 2], "i": [0, 1, 2]}
    df = nw_frame(data, nw).lazy()
    reveal_type(df)  # -> LazyFrame[Any]

    native = df.to_native()
    reveal_type(native)  # -> Any
    
    frame = nw.from_native(native)
    reveal_type(frame)  # ->  Any

which seems far from ideal! I didn't manage to get the __call__ overload quite right, but it should not be far from the correct solution. I tried a few other variations without success.

---

Regarding the patches in conftest.py:

1. I wouldn't worry too much for their typing, they are just temporary workaround to avoid touching every single test
2. The hack is needed for the same reason, namely make narwhals (main namespace) the default namespace to the __call__ method and avoid having to pass it everywhere. This works since nw.from_native can also take a narwhals object