feat: adding components of the datacite metadata model

[markdoerr]

DataScite has a nice collection of Metadata that they add to their DOIs:

https://datacite-metadata-schema.readthedocs.io/en/4.5/properties/overview/

Let's discuss, which of these are relevant for us.

[dalito]

I went through all relations defined in datacite schema version 4.5 when I created the enum RelationType. So at least this aspect should already be well reflected.