Consider setting MSBuild properties to inform about the presence/absence of global.json in a repo

### Is your feature request related to a problem? Please describe.

If a build logic author wants to enforce a global.json in a repo, the only mechanism they have today is probing and parsing for it on their own. The CLI already inherently does this - it should provide properties similar to the ones that the SdkResolver does so that there's a uniform expectation of availability.