Support for XR SDK and incremental improvements

• Improved support for Unity 2020.
  • Auto-migration tested and working.
• Improved support for XR SDK.
• Support direct coding to ARCore.
  • See sample here.
• Space Pin visualization (awesome community contribution).
  • See sample usage here.
• Update to MRTK v2.5.4.
• Some advance fixes from MRTK v2.6.0.
• Internal anchor density options to improve performance in very large spaces.
  • See notes here.
• Support for iOS as well as HoloLens and Android, via XR SDK.
  • See setup instructions here.
  • Full iOS support waiting on a FrozenWorld Engine release, expected in the next few days.
• Minor bug fixes and documentation improvements.