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]

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%