[feat]: allowing all fields to be set as default for `FromPyObject`

In https://github.com/PyO3/pyo3/pull/4829#issuecomment-2577336141, FromPyObject was intentionally designed to disallow all fields being set with #[pyo3(default)]. I wonder if this is because:

ref: https://github.com/PyO3/pyo3/pull/4829#issuecomment-2572971732

the surprising behavior that a variant Foo { #[pyo3(default)] value: usize } becomes a catch-all just like the Foo {} variant.

However, for me, this behavior makes sense, as I use it to extract class Foo(TypedDict, total=False) for typed kwargs.

// 💥 cannot derive FromPyObject for structs and variants with only default values
#[derive(FromPyObject)]
#[pyo3(from_item_all)]
pub struct Foo {
    #[pyo3(default)]
    id: Option<i32>,
}

#[pyfunction]
#[pyo3(signature = (**kwargs))]
fn foo(kwargs: Option<&Bound<'_, PyDict>>) -> PyResult<()> {
    let foo: Option<Foo> = kwargs.map(|kwargs| kwargs.extract::<Foo>()).transpose()?;
    Ok(())
}

from typing_extensions import NotRequired, TypedDict, Unpack


class Foo(TypedDict, total=False):
    id: NotRequired[int]


def foo(**kwargs: Unpack[Foo]) -> None: ...

foo()  # ok
foo(id=1)  # ok
foo(id=1, foo=2)  # Err: no `foo` parameter

The entry points named are FromPyObject, #[pyo3(default)], and #[pyo3(from_item_all)]; start by tracing how the derive handles structs or variants whose fields are all defaulted, using the linked PR discussion for the existing catch-all rationale. Done means the all-default Foo example derives and extracts optional id values while still rejecting unknown keys.