Support MSBuildWorkspace.OpenProjectAsync() with file-based program

[RikkiGibson]

See also dotnet/roslyn#78303 (comment)

Eventually people will want to be able to use MSBuildWorkspace to load a file-based program. At minimum we probably want to add an OpenProjectAsync which takes both a project path and content, so, if the caller is able to put together a virtual project, they can make a workspace for it.

We may also want a new method that takes the FBP path itself and makes the virtual project and so on under the hood.