add anchoring extension to sofa glTF

[jordijoangimenez]

Hi @Etiennefaivredarcier, I've merged the original PR from @nlsdvl which includes this furniture: https://github.com/5G-MAG/rt-xr-content/blob/development/furnitures/scene.gltf

I've merged it without waiting for your review so I can now bring all the updates to the repo to classify the assets.

We'd like you to add the anchoring extesions to this glTF, as it is just without them.

Thanks!

[nlsdvl]

@jordijoangimenez for this use case, not only do we want the model to be anchored, but also we want to be able to position it in space. cc @Etiennefaivredarcier