rust: auxiliary: add auxiliary registration

Implement the `auxiliary::Registration` type, which provides an API to
create and register new auxiliary devices in the system.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250414131934.28418-5-dakr@kernel.org


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