rust: hrtimer: implement `HrTimerPointer` for `Pin<Box<T>>`

Allow `Pin<Box<T>>` to be the target of a timer callback.

Acked-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20250309-hrtimer-v3-v6-12-rc2-v12-10-73586e2bd5f1@kernel.org


Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>