Create WindowsDesktop + .NET Core support bundle

[dagood]

Similar to the "ASP.NET Core/.NET Core Runtime & Hosting Bundle", there should be one for WindowsDesktop that sets up everything needed to run a WPF or WinForms app on .NET Core. It should include both x86 and x64, and both the .NET Core Runtime and Windows Desktop Runtime.

[dagood]

/cc @leecow, reference https://github.com/dotnet/core-setup/issues/7530#issuecomment-522355716 for the list of installers I think will exist once this is done. Branding is discussed more broadly in dotnet/windowsdesktop#191 as well.

[dagood]

We have new plans in 3.1 to use a single installer with options to install various components including this runtime, with dependency information to simplify the process. dotnet/installer#5019

With that plan, it's unclear whether this artifact is necessary--closing for now.

[dagood]

There's some uncertainty about the 3.1 installer fulfilling this scenario, so I'm adding some detail about what makes this difficult:

The x86 and x64 installers are built in parallel on different machines. However, all the MSIs need to be present on a single machine during the official build to bundle them. There are two main ways I have on my mind to do this:

1. Add a "join" step to the build, where the MSIs are all downloaded from the parallel legs and assembled into the bundle.
   • This seems fairly low risk, because the join part of the build would be fairly isolated from the existing build.
2. Build x86 and x64 on the same machine.
   • I don't think this is a useful approach for the current situation.
     • I believe ASP.NET Core's Support Bundle uses this approach, but they have had time to validate it and used it for a while.
   • I have concerns that this will give us bad results (some build outputs not being differentiated per arch), and this adds considerable risk.
   • This also would roughly double the product build time.

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There is an alternative that skips that... produce one support bundle per arch. Users would need to figure out whether they need x86 or x64, or install both, but it would be quick to implement in Core-Setup.

I've filed https://github.com/dotnet/core-setup/issues/8504 to track this incremental bundling step.