Enhance unused reference feature to support undo/redo

We would like the API implemented in https://github.com/dotnet/project-system/pull/6692 enhanced with the ability to re-add a reference that has been removed.

One possibility is a new UpdateAction.Add In particular the call to IProjectSystemReferenceCleanupService.GetProjectReferenceAsync would need to be enhanced so that we capture enough information about each reference that it could be recreated from the ProjectSystemReferenceInfo model. This information may need to include the position in the project file where the reference would be inserted. Next the IProjectSystemReferenceCleanupService.TryUpdateReferenceAsync would need to be updated to support an ProjectSystemUpdateAction.Add scenario where, given this enhacned ReferenceInfo model, a reference is added to the project file.

Another possibility is that we share the same undo context The IProjectSystemReferenceCleanupService.TryUpdateReferenceAsync could return a OleUndoUnit or we could pass in a UndoTransaction of sorts.

Tracked in DevOps 1348735

[JoeRobich]

@jjmew It would be coarse-grained but we could have the proposed Undo() and Redo() methods return bool indicating success. We would then inform the user on our end.

[drewnoakes]

@ocallesp do you recall which version this was fixed in?