Define glTF Object Model properties for OMI extensions

[aaronfranke]

Draft because this depends on https://github.com/omigroup/gltf-extensions/pull/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.