Publish OpenApiDiff to Nuget (or make dotnet cli tool)

Having this tool as a node package is great... but having it as a nuget package or dotnet CLI tool would be even better for those of us who have API's without javascript frontends. Otherwise I need to add node to my build pipeline in order to use it.

Start by inspecting the existing Node package and the command-line entry point for OpenApiDiff. Determine how a NuGet package or .NET CLI tool would be built and published without requiring Node; done means users can install and run it from a .NET-based build pipeline and the published artifact is documented.

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