Support for XR SDK

• Persist local anchors on HoloLens via XR SDK/MR OpenXR extensions.
  
• 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 weeks.
    
• Minor bug fixes and documentation improvements.