Support other metadata schemas

Meta-issue for support for other metadata schemas (Croissant, MLDCAT-AP, ...).

We had a somewhat functional implementation for MLDCAT-AP, this was removed in (92c6f3b).
While we want to bring it back later into the development, for now we do not want to concern with keeping that code up-to-date, and we can re-evaluate how we support other schemas more generally before we bring it back.