Enum variant relations

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

Use enum variants to define relations between entities.

### Example: graphical editor

Set of 2D objects in parent-child hierarchy with components: Color, Shape, Transform.

Color could be a single color OR inherited from another (probably ancestor) entity.

We may also need many more possibly inherited attributes like stroke width, fill, etc.

## What solution would you like?

Make it possible to define them like this:

```rust
#[derive(Component, Debug)]
#[relationship_target(relationship = Color::Inherited)]
pub struct ColorInheritedBy(EntityHashSet);

#[derive(Component, Debug)]
pub enum Color {
Value(bevy::color::Color),
#[relationship(relationship_target = ColorInheritedBy)]
Inherited(Entity),
}
```

Relationship is added when Color::Inherited is added to an Entity.

The macros could have a different name of course if implementation is not feasible to share with the regular relationship definitions.

## What alternative(s) have you considered?

Could just store color value in a component and relation in another one.

## Additional context

I wonder what you think about this kind of design. Enums are nice.

Start by reviewing Bevy's existing regular relationship definitions and the relationship and relationship_target macro entry points mentioned in the proposal. Compare the enum-based Color example with the alternative of separate components, then determine the intended API and acceptance criteria before implementation.

Written by the indexing model from the issue text.