Enable ASP.NET Core app to share auth/identity decisions with ASP.NET app

Current tooling allows incremental migration to share ASP.NET authentication with an ASP.NET Core app for quick startup moving endpoints.

In addition to this existing functionality, it would be useful to be able to have the ASP.NET Core authenticate users and forward identity information to the ASP.NET app (in cases where it's necessary to fall back to the ASP.NET app to handle a request). This would allow developers to migrate authentication at any point in the migration process that was convenient instead of having to wait until all ASP.NET endpoints have been migrated first.