[FEATURE REQUEST] Add Mac support

[bd-jimmy]

Add support for using a Mac as a development machine for MRTK projects

First of all the installer/setup program is windows only. When I try to run the project scenes on a Mac, I get a bunch of console errors that say the Microsoft OpenXR plugin is not loaded. In addition, there are some windows specific controls and features that get in the way if you try to run on mac e.g. windows speech, tts handler etc.

If we port this to Mac, it can more easily enable other feature requests too, like #88

Describe the solution you'd like

The installer/setup program should work on Mac as well. It should setup the project so that people can start it up and run all the scenes on a Mac. In addition, the project repo should be in a state that can be run on Mac if someone downloads the main branch.
All features that do not work on Mac should be disabled cleanly. Project Validation should be setup to check that.

Describe alternatives you've considered

Get a Windows machine. I have tried to manually remove components and fix others and I can get things to run, so I can maybe help this effort along by putting in a MR.

Additional context

Many mobile developers use Macs and it would be great to get them to use MRTK and get more engagement.

[shaynie]

We're open to community contributions to enable a better mac development experience. Once the packages are available in UPM (bug #518) we won't be relying on the Mixed Reality Feature Tool for bringing in packages.