Multiple `AnimationTarget` per entity

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

Currently, an entity can only have a single `AnimationTarget` because it needs to be inserted directly into the entity. This reduces flexibility when you want to have multiple, decoupled animations per entity. For example, an entity can have an idle animation playing by default. And when the entity gets attacked, you want to insert another animation that tweens the entity's color so that it flashes red.

## What solution would you like?

Allow `AnimationTarget` to be a child of the target entity. This allows an entity to have multiple `AnimationTarget`s as children.

## What alternative(s) have you considered?

I don't think there's a way to work around this limitation with the built-in animation system.

## Additional context

The approach taken by https://github.com/Multirious/bevy_tween is that animators can be the child of the target entity. The animator will traverse up the hierarchy to find the target entity. This means than an entity can have multiple active animations.