Create a tool and msbuild task to find differences in nuget packages

[jtschuster]

Creates a command line tool and an msbuild tooltask to find differences in nuget packages. It does a git-style diffing to show line by line differences in the list of files in the package and in the nuspec file. There isn't yet a way to handle expected differences, so the msbuild task always succeeds, but will log any differences it finds.

I'm planning to do more in depth checks on each assembly in the packages and check for each member on each type and take a list of assemblies with expected differences to skip these checks.

Part of dotnet/source-build#4051

[ViktorHofer]

Why arcade? I would have assumed this tool to be added to dotnet/dotnet (-> dotnet/installer) under tools. That's the place where we have our other msbuild tasks that we then use to analyze the product, i.e. pre-built detection, poison detection, etc.

[jtschuster]

Why arcade?

No reason in particular, this just came to mind first. I'll move it.