Conditional configuration of restore settings used by NuGet SDK resolver

Follow up to https://github.com/Microsoft/msbuild/issues/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](https://github.com/NuGet/Home/issues/3972). Instead, users are instructed to use `/target:Restore` and [MSBuild properties](https://docs.microsoft.com/en-us/nuget/schema/msbuild-targets#restore-target) 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`.

Start by reading the NuGet SDK resolver entry points and the /t:Restore restore-target documentation mentioned in the issue. Determine how restore settings are supplied during SDK resolution and how they are conditionally supplied by MSBuild; done means both paths support consistent conditional restore configuration.

Written by the indexing model from the issue text.