Native Unity plugin for Windows UIAutomation API

[david-c-kline]

Investigate and prototype a native plugin for Unity that responds to the Windows UIAutomation API for accessibility.

[mwcampbell]

@davidkline-ms I think we should collaborate on this. @DataTriny and I are working on a prototype Unity plugin for making games accessible with our AccessKit project. AccessKit is only implemented on Windows so far (using UIA), but a Mac implementation is coming sometime this year, and hopefully an implementation for at least one mobile platform as well. AccessKit uses a push-based approach that I think is particularly well-suited for cases like this where minimizing the overhead of a foreign function interface like P/Invoke is important, because the Unity plugin can push one serialized accessibility tree per frame to the native code.