Add PackageDiff tool and task

[jtschuster]

Package Diff tool for https://github.com/dotnet/source-build/issues/4051

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.