Conditional configuration of restore settings used by NuGet SDK resolver

[natemcmaster]

Follow up to #2803

The NuGet SDK resolver currently only supports using NuGet.config for restore settings. NuGet.config is not expressive enough to allow conditionally changing settings, and there are currently no plans to change that. Instead, users are instructed to use /target:Restore and MSBuild properties to conditionally configure restore settings. However, these properties are not available during the SDK resolution phase, so they cannot be used to control the restore used by the NuGet SDK resolver.

It would be great to provide ways to control restore settings in way that is consistent between the NuGet SDK resolver and /t:Restore.

[dasMulli]

It would be great to allow env vars or command line settings (-> Directory.Build.rsp) to override the nuget config used and/or specify sources.

[weshaggard]

On top of configuring the source we need a way to configure the version that doesn't require it to be hard-coded in the projects or global.json. There are scenarios like source-build where we build all the repo's that make up the .NET Core product and they need to use a version of an sdk that was built from source and so we need a way to tell all the repo's to use that version without necessary committing or overwriting changes in every repo.