rust: add `ZeroableOption` and implement it instead of `Zeroable` for `Option<Box...

rust: add `ZeroableOption` and implement it instead of `Zeroable` for `Option<Box<T, A>>`

When making pin-init its own crate, `Zeroable` will no longer be defined
by the kernel crate and thus implementing it for `Option<Box<T, A>>` is
no longer possible due to the orphan rule.
For this reason introduce a new `ZeroableOption` trait that circumvents
this problem.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250308110339.2997091-11-benno.lossin@proton.me
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>