Define glTF Object Model properties for OMI extensions

[aaronfranke]

Draft because this depends on #226 (they would otherwise conflict, so I had to choose one to go first).

This PR adds glTF Object Model property definitions for all OMI extensions, except OMI_physics_joint (pending rework) and OMI_collider (archived).

Note that this PR is not the be-all end-all of OMI's support for the glTF Object Model. It is also important that we define things like how triggers interact with interactivity, or how one might play audio or apply a force. However, this PR is the start, defining at least the properties, which will work for KHR_animation_pointer, and already have known established best practices. Also, for physics, this aligns with Eoin's Microsoft/KHR physics repo which has these already.

[aaronfranke]

Merging this without a review because:

• Defining the object model properties is required to move our specs forward, Khronos expects that extensions define how they interact with the object model for use with things like KHR_animation_pointer and KHR_interactivity.
• We need to get this in or else our PRs will conflict with each other, because this touches every spec a little bit.
• Nothing here is changing the existing behavior, it is just formally defining how the existing properties in our specs can be accessed by things like KHR_animation_pointer and KHR_interactivity. Therefore, there is nothing controversial in here, it is simply an upgrade in formal definition.

Even though this is merged, review is still very welcome and encouraged. We can improve things in future PRs as needed.