`cfg_attr` in bridge module

[NiceneNerd]

I have a project using cxx to which I wished to add serde support, but I would prefer it be an optional feature. Several of the types exposed by its API are shared types declared in the FFI bridge. Unfortunately, it seems that cfg_attr is not currently supported within the bridge. Example:

    #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
    #[derive(Debug, Default, Clone, Copy, PartialEq, PartialOrd)]
    pub struct Vector2f {
        pub x: f32,
        pub y: f32,
    }

Which receives simply an Unsupported attribute error.

I have no idea how difficult or not it would be to accommodate this use case, but if not too complex I would find it quite helpful. Otherwise I may need to make some wrapper types or some other workaround.