Support setting custom functions for `FromReflect` impl

[davidpdrsn]

For validating fields or maintaining invariants.

API could be something like

#[derive(Reflect)]
struct Foo {
    #[reflect(from_reflect_with(clamp_ratio))]
    ratio: f32,
}

fn clamp_ratio(ratio: &dyn Reflect) -> Option<f32> {
    Some(ratio.downcast_ref::<f32>()?.clamp(0, 1.0))
}