extended AuthenticateResponse w/Meta 'DisplayName'

Relates to [this commit](https://github.com/ServiceStack/ServiceStack/commit/242de40773022aca0d90e85303691d010e06a36d)  The problem is that many OAuth provider do not have notion of userName, and we have name-less auth details. With "DisplayName" meta property clients (like Single Page Apps) would be able to provide more personalized experience (e.g. in login/logout UI elements)