rust: page: define trait AsPageIter

The AsPageIter trait provides a common interface for types that
provide a page iterator, such as VmallocPageIter.

Subsequent patches will leverage this to let VBox and VVec provide a
VmallocPageIter though this trait.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://lore.kernel.org/r/20250820145434.94745-5-dakr@kernel.org


Signed-off-by: Danilo Krummrich <dakr@kernel.org>