Adding JSON to specify PowerFx functions support

[cwduncan]

Pull Request Template

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Checklist

• [ ] The code change is covered by unit tests. I have added tests that prove my fix is effective or that my feature works
• [ ] I have performed end-to-end test locally.
• [ ] New and existing unit tests pass locally with my changes
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I used clear names for everything
• [ ] I have performed a self-review of my own code

[github-actions[bot]]

Package	Line Rate	Branch Rate	Complexity	Health
Microsoft.PowerApps.TestEngine	91%	87%	982	✔
Summary	91% (2532 / 2792)	87% (596 / 684)	982	✔

Minimum allowed line rate is 85%