samples: rust: add Rust auxiliary driver sample

Add a sample Rust auxiliary driver based on a PCI driver for QEMU's
"pci-testdev" device.

The PCI driver only registers an auxiliary device, in order to make the
corresponding auxiliary driver probe.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250414131934.28418-6-dakr@kernel.org


[ Use `ok_or()` when accessing auxiliary::Device::parent(). - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>