Add the object hit to the XRHitResult structure

[lincolnfrog]

In order to support more intelligent logic around hit results and anchors, it would be very useful to have the object that was hit (plane, mesh, etc.) included in the hit structure. This task requires some definition of world understanding elements and collaboration with the anchor API in order to make sense.

[blairmacintyre]

I would like us to eventually return more details of the world, both with hit test and potentially more automatically. BUT I don't think we can until we figure out what kind of permissions we will ask of the user to get informed consent.

I'm not in favor of returning those details here with broader consent.

For example, if the user has given permission for world understanding data to go into the web page, then we can pass those in as they are discovered, and just include a reference here (i.e., each element would have a unique id, and then the hit can just include the id)