rust: configfs: introduce rust support for configfs

Add a Rust API for configfs, thus allowing Rust modules to use configfs for
configuration. Make the implementation a shim on top of the C configfs
implementation, allowing safe use of the C infrastructure from Rust.

Link: https://lore.kernel.org/r/20250508-configfs-v8-1-8ebde6180edc@kernel.org


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