Allow required components to specify relationships

[Freyja-moth]

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

As mentioned in #19045 and #16840 it would be nice to have a way to specify relationships in required components

What solution would you like?

Perhaps something like this?

#[derive(Component)]
#[require(
    Related::<Children>((
        Text::new("Some interesting..."),
        (
            Text::new("writing here."), 
            TextColor(css::PURPLE)
        )
    ))
    // Not sure if this should override or add to the parent if it already exists, leaning to add, but depends on feasibility and what other people want
    Related::<ChildOf>(
        Node
    )
    Related::<Observer>((
        // This gets more tricky because observers are m-m, not sure how to do this without duplicating a lot of observers that do the exact same thing.
    ))
)]
pub struct TextContainer;

What alternative(s) have you considered?

Use the old method, but that doesn't hold up relationship invariants.