Sphere preview of content

[Toys0125]

Summary

• add an opt-in storage setting for shared content previews
• render live avatar/prop/world previews inside content share spheres
• improve preview material fallback handling for generic renderers
• update the avatar orb assets to better present shared previews
• keep the default behavior unchanged by leaving previews disabled by default

Testing

• Ran in Unity and did testing with setting on and off. Does not retro-active load previews for content shares already spawned.

[Toys0125]