rust: device: introduce device::CoreInternal

rust: device: introduce device::CoreInternal

Introduce an internal device context, which is semantically equivalent
to the Core device context, but reserved for bus abstractions.

This allows implementing methods for the Device type, which are limited
to be used within the core context of bus abstractions, i.e. restrict
the availability for drivers.

Link: https://lore.kernel.org/r/20250621195118.124245-2-dakr@kernel.org
[ Rename device::Internal to device::CoreInternal. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>