Add method to check PyCapsule name while getting pointer

[wjones127]

PyCapsule::pointer() passes the name of the PyCapsule itself, instead of passing a user-provided name. This means users must perform the validation of the PyCapsule name themselves.

let capsule: PyCapsule = object.extract(py)?;
if capsule.name() != expected_name {
    return PyValueError::new_err("Capsule does not have expected name");
}
let pointer = capsule.pointer() as *mut MyDataType;
if pointer.is_null() {
    return PyValueError::new_err("Capsule pointer was null");
}
// do something with pointer.

It might be nice to instead have an API that is more strongly typed and convenient:

let capsule = unsafe { TypedPyCapsule::<MyDataType>::from_object(obj, expected_name)? }; // does name check
let pointer: NonNull<MyDataType> = capsule.get_pointer()?; // does null check

[wjones127]

Substantial discussion was had in this PR: #3585

I'll implement this as a utility wrapper in upstream projects and meanwhile I'll let the project maintainers here think about whether this is something that would make sense in PyO3.