ToolsReference and Multi TFM Support in NuGet UI

Issue

Link to the GitHub issue that tracks the work and discussion.

ToolsReference : https://github.com/NuGet/Home/issues/3505

PackageReference UI: https://github.com/NuGet/Home/issues/3457

Problem

With the advent of .NET Core, we need to support addition of ToolsReferences from within the NuGet package manager and PMC. In addition, we also need

Who is the customer?

.NET Core customers for ToolsReferences and all .NET customers in VS for PackageReference

Solution

ToolsReferences