Add support for references between entities.

[andriyDev]

Fixes #187.

The issue has more context on the idea behind this PR.

[andriyDev]

I am leaving a TODO for the docs, examples and tests. I will get around to that soon.

For a short walkthrough of what this looks like in practice, users just have to:

1. Add the GltfRefMapPlugin.
2. Implement FromGltfRef for each type.
3. Add the GltfRefPlugin<T> for each type.

bevy_registry_export can export the registry to Blender which will include:

• GltfRefTarget - Holds a string that can be used to identify this entity.
• GltfRef<T> for each T - Holds a string that can be used to pick which entity should be associated with T on this entity.

[andriyDev]

I realized I should be using the SceneInstance component to determine where Gltf roots are, hence the force push.