add viewnode to ar-model-viewer sample

[sameerjj]

ViewNode is in high demand in terms of library functionality, it seems. Putting ViewNode in the sample code A) shows users how to use this functionality B) makes it a core part of testing of the library

[sameerjj]

PS I wasn't able to test this addition unfortunately. That's because this sample code is already crashing. I wasn't able to see any glaring crash issues that would be helpful, but here are the logs: ar-model-viewer-crash.txt

[rawello]

hello, i try use that sample in my project, but i get "black hole", i try build your sample - and same i get black hole

https://drive.google.com/file/d/1Dd8VmHjhMH-2BypYYFED9OGxy9_B_Qha/view?usp=drivesdk