Add dotnetup as an optional resolver for missing .NET SDKs

[kpobb1989]

Have you considered making dotnetup an optional dependency of the dotnet CLI? If a required SDK like .NET 8.0 is not installed, the CLI could automatically resolve and install it via dotnetup, without requiring manual user intervention.

[nagilson]

Eventually we might put it directly into the CLI, but right now we are trying it out as a preview to see if folks like it or not. If its well-received then we will be looking into that option.