[PackageReference] Centrally manage NuGet package versions for a solution or a repo

PackageReference enhancements are summarized as part of this Epic issue: https://github.com/NuGet/Home/issues/6763 

* Status: **Incubation**
* Spec: [Centrally manage NuGet package versions for a solution or a repo](https://github.com/NuGet/Home/wiki/Centrally-managing-NuGet-packages)