UI specific color enum

[ickshonpe]

What problem does this solve or what need does it fill?

Bevy UI's visual components like BackgroundColor, BorderColor, TextColor and ImageNode only take a flat Color and there is nowhere to add support for other color effects.

Related #18139

What solution would you like?

Replace the UI Color fields with a UiColor enum:

pub enum UiColor {
    Color(Color),
    /// Once gradients support is added
    Gradient(Gradient),
}

UiColor would implement the Color conversion traits so that code like:

commands.spawn((Node::default(), BackgroundColor(ORANGE_200.into())));

would remain unchanged. And to draw a node with gradient you'd use:

commands.spawn((
    Node::default(),
    BackGroundColor(LinearGradient::to_left(vec![Color::WHITE.into(), ORANGE.into(), (BLUE_200.into(), Val::Px(50.)).into()]).into())
));

What alternative(s) have you considered?

I don't see any viable alternatives.

Additional context