Initialize ARFoundation session

[FejZa]

XRTK - Mixed Reality Toolkit Feature Request

Is your feature request related to a problem? Please describe

Opening a new feature request as part of the general Arfoundation support discussed in #170. The first step to have ARFoundatin working is to properly Setup the session.

How would you classify your suggestion

• New platform support

Describe the solution you'd like

For ARFoundation to work, the following steps Need to be implemtend:

• [ ] Create ARSession component when running with ARFoundation enabled. It can be added to any GameObject. Fort XRTK preferably to the playspace GO
• [ ] Create ARSessionOrigin component, probably also best added to the playspace GO
• [ ] Add TrackedPoseDriver to main camera
• [ ] Implement to optionally add ARCameraBackground to the main camrea GO so the video feed is rendered to the background

Additional context

https://docs.unity3d.com/Packages/com.unity.xr.arfoundation@1.0/manual/index.html

[StephenHodgson]

After some of the announcements from Unite I'm not sure I want to try and support ARFoundation. Likely there's gonna be conflicts between the XRTK and unity's XR offerings.

[FejZa]

@StephenHodgson from what I have learned since I created this PR personally I don't see how / why XRTK should support ARFoundation, since they are somewhat converging to become "competitors". ARFoundation is currently a subset of what XRTK already is and just an abstarction layer. IMHO this PR is obsolete.