Pinning Addin Version

If addins are referenced through a packages.config in the addins folder, file they can be pinned the same way as the Cake version (when using the Cake Runner for .Net Framework or .NET Core).

If addins are referenced using the #addin preprocessor directive they can be pinned like this:

#addin nuget:?package=Cake.Foo&version=1.2.3