Making MRTK display content better across AR and VR experiences

[RogPodge]

Hey all, there's been a recent proposal for creating a MRTK-centric MixedRealityContent object and associated system to better align the scene content across AR and VR experiences.

#9512

In summary:
The main point of the MixedRealityContent object is to guide the users to laying out their content in a certain hierarchy, and to provide a place for us to instantiate new MRTK objects and ensure they are aligned to the user experience settings (the floor height and experience scale)

We're looking for feedback and discussion from the community regarding this approach.