AuthenticationTransformer

Transforms authentication information to a new type, for use in combiners. If transformers are available for A, B, and C, the combined authentication can use simple type D instead of something like (Option[A], Option[B], Option[C]).

Implementation note: This is a new trait instead of a type alias to prevent accidental implicit use. It's sealed to control how partial is defined to make future refactorings easier (maybe it should be a complete lifted function, etc.).