Expose SingleRuleSupportedValuesProvider as a public type

## Summary

MAUI has a couple of scenarios where they need to expose MSBuild items through an `IDynamicEnumValuesProvider` so that they can show up as options in a drop-down menu in the project property pages. We do something very similar for lists of platforms and OS versions, with most of the logic pulled out into [SingleRuleSupportedValuesProvider](https://github.com/dotnet/project-system/blob/main/src/Microsoft.VisualStudio.ProjectSystem.Managed.VS/ProjectSystem/VS/Frameworks/SingleRuleSupportedValuesProvider.cs) and [SupportedValuesProvider](https://github.com/dotnet/project-system/blob/main/src/Microsoft.VisualStudio.ProjectSystem.Managed.VS/ProjectSystem/VS/Frameworks/SupportedValuesProvider.cs).

We should document these types and expose them publicly so that others can reuse this logic (which can be rather complicated) rather than copying it into their own code or writing their own version.