Structure for Additional Information about Learners

We are reaching the part where LMS needs to store additional information about learners besides just the learner model. The sparking idea was that we probably want to keep both IDM logins as well as email if you change from one type of account to the other, independently of which of those is actually the "primary identifier". (VoLl-KI @ FAU Jour Fixe 2024-04-08)

This needs some thought on how best to do this so it doesn't get too complicated while remaining flexible and extensible.