Recover and forward AsyncTaskUtil to ExcelAsyncUtil

[govert]

See this discussion: https://groups.google.com/g/exceldna/c/9VtfJ6SkhdU

I think it's worthwhile to try to keep compatibility of the public interface of ExcelDna.Registration, where easy and not interfering with our refactoring plans. It would be good if code using ExcelDna.Registration continued to work without changes, maybe with 'Obsolete' attributes added to indicate the change.

In particular, we might restore the class and public interface of ExcelDna.Registration.Util.AsyncTaskUtil, to forward all calls to the ExcelDna.Integration.ExcelAsyncUtil class, and have some Obsolete attributes to indicate to recommended update path.

[Sergey-Vlasov]

Maybe we should mark the whole ExcelDna.Registration package as Obsolete instead of marking individual methods once I complete the migration?

[govert]

Maybe we should mark the whole ExcelDna.Registration package as Obsolete instead of marking individual methods once I complete the migration?

Yes, that sounds good.