Create cgmanifest.json file

[wtgodbe]

This file should list all the dependencies in the repo that don't get restored by the regular repo build (and there don't get scanned by CG) - we think the most likely source of such dependencies is templates.

Example file
Docs

[marcpopMSFT]

This was filed here to track the test templates which include PackageReferences. If the templates include references to packages that are already scanned in the build, you don't need to do this. Basically, CG only scans what is built and what's in the nuget cache so if you don't build or run these templates (or build the packages referenced in them) then you may need a cg manifest.