rust: pin-init: add `Zeroable::init_zeroed`

The trait function delegates to the already existing `init_zeroed`
function that returns a zeroing initializer for `Self`.

The syntax `..Zeroable::init_zeroed()` is already used by the
initialization macros to initialize all fields that are not mentioned in
the initializer with zero. Therefore it is expected that the function
also exists on the trait.

Link: https://github.com/Rust-for-Linux/pin-init/pull/56/commits/a424a6c9af5a4418a8e5e986a3db26a4432e2f1a
Link: https://lore.kernel.org/all/20250523145125.523275-3-lossin@kernel.org


Signed-off-by: Benno Lossin <lossin@kernel.org>