Support `#[derive(IntoBytes)]` on unsized types

*See also: https://github.com/google/zerocopy/issues/2063#issuecomment-2672892894*

Currently, the code generated by our `IntoBytes` derive includes calls to `core::mem::size_of` in order to determine whether a type has padding (unless the type is `repr(transparent)` or `repr(packed)`). This prevents deriving `IntoBytes` on unsized non-`repr(transparent)`/`repr(packed)` types.