Expose events via OAI-PMH endpoint

**Is your feature request related to a problem? Please describe**

Events can not be exchanged through OAI-PMH.

**Describe the solution you'd like**

Expose events through OAI-PMH similar to materials.

**Describe alternatives you've considered**

Alternatively events can be ingested e.g. through the Bioschemas ingestor.

**Additional context**

This is relevant for the mTeSS-x project.
