rust: workqueue: add support for ARef<T>

rust: workqueue: add support for ARef<T>

Add support for the ARef<T> smart pointer. This allows an instance of
ARef<T> to handle deferred work directly, which can be convenient or even
necessary at times, depending on the specifics of the driver or subsystem.

The implementation is similar to that of Arc<T>, and a subsequent patch
will implement support for drm::Device as the first user. This is notably
important for work items that need access to the drm device, as it was not
possible to enqueue work on a ARef<drm::Device<T>> previously without
failing the orphan rule.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://lore.kernel.org/r/20260323-aref-workitem-v3-1-f59729b812aa@collabora.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>