Allow configuring sources as authentication required or preferred

[nkolev92]

NuGet Product(s) Involved

NuGet.exe, Visual Studio Package Management UI, MSBuild.exe, dotnet.exe

The Elevator Pitch

Today, the NuGet tooling relies on a 401 challenge to understand whether a source needs auth.
This adds a roundtrip for a few of the urls hit the client.

It would be nice, if NuGet would just try to acquire credentials for certain feeds as marked by the user.

This would mean no wasted requests and allow for the source to cleanly know the difference between an authenticated and unauthenticated user from the onset and maybe build some experiences of off that.

Additional Context and Details

The behavior today, only as needed works great when a feed is either fully public, no 401 challenges, or fully private, 401 challenge on every url, but it's not really smooth for sources that may for example be only partially public.

When I'm calling the source public/private, I'm only referring to the restore side. Push tends to require authn even on feeds that are public for restore.

[zivkan]

More or less a duplicate of #11600. If there are differences between the two issues, I think they can be exaplained as a comment, rather than a new issue.

[zivkan]

Apologies to @nkolev92. In the end I'm going to create an epic to track several different work items, as the implementation is becoming more complex, and this ("sub")feature request has different challenges that other parts of the feature.