Create nuget package diff tooling

[mmitche]

Create a tool and msbuild task to find any differences between nuget packages produced by the VMR and the official build.

• File structure
• Nuspec properties
• Assembly versions
• Handle expected differences
• Wire up package diffing tool into official build CI #4106

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[tkapin]

Is this a duplicate of #3819?

[mmitche]

Subitem in the epic.

[mmitche]

@jtschuster update as necessary