`#[derive(uniffi::Record)]` should support `#[uniffi::skip]` attribute to skip fields

[oriongonza]

Right now the only way to pass the struct through uniffi with different fields is to create a "close but not quite" struct with the missing fields.

Serde has lots of custom attributes to control deserialization, uniffi could support a few of those. #[uniffi::skip] is almost trivial to implement.

This doesn't make much sense for Object or Enum, though.