rust: pin-init: Implement `InPlaceWrite<T>` for `&'static mut MaybeUninit<T>`

rust: pin-init: Implement `InPlaceWrite<T>` for `&'static mut MaybeUninit<T>`

This feature allows users to use `&'static mut MaybeUninit<T>` as a
place to initialize the value. It mirrors an existing implemetation
for `Box<MaybeUninit>`, but enables users to use external allocation
mechanisms such as `static_cell` [1].

Signed-off-by: Oleksandr Babak <alexanderbabak@proton.me>
Link: https://crates.io/crates/static_cell
[ Added link to `static_cell` - Benno ]
Signed-off-by: Benno Lossin <lossin@kernel.org>