rust: alloc: allow coercion from `Box<T>` to `Box<dyn U>` if T implements U

rust: alloc: allow coercion from `Box<T>` to `Box<dyn U>` if T implements U

This enables the creation of trait objects backed by a Box, similarly to
what can be done with the standard library.

Suggested-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250412-box_trait_objs-v3-1-f67ced62d520@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>