VS project nominations should use generic data structure

[zivkan]

For project systems in Visual Studio that support both CPS and PackageReference capabilities (so in effect just the SDK style .NET projects) use a NuGet data flow where the project system pushes project item and properties to NuGet via an API called NominateProjectAsync.

This API has a data structure where the interface has properties ProjectReferences, PackageReferences, and so on. So, every time NuGet wants to add a new MSBuild item, we need to create a new iterface for the new item type, publish the package, then edit dotnet/project-system to use the new package and set the new property for the item.

Project properties and item metadata already use a generic property bag, which is more or less equivalent to IReadOnlyDictionary<string, string> (but is different for COM compatibility). Items should also use a generic data structure, so that future additions to which MSBuild items NuGet reads no longer requires project system to update package versions to implement the change.