Add global.json

[tamararivera]

This change makes the global.json file mandatory and its content controlled by GitHubSync

[DavidBoike]

Can't think of a reason not to, but I'd love to make sure Brandon can't think of anything either before merging.

[tamararivera]

Or if he has something that we could add for this round

[bording]

This has been discussed some, and at this point I don't think this is something we want to do. There are times where we might want to be flexible in SDK roll out.

Or there might be other things that we need to selectively add there. For example, if/when we roll out Microsoft.Testing.Platform, we need to add a setting to global.json, and we might not want to make that change to all release branches also.

Going to go ahead and close this for now.