Implement event validation in EventProcessorEngine

## Description

We expect an `EventProcessor` to set certain fields on an event. To make sure created events are valid, we have to implement an event validator that will be called in the `EventProcessorEngine`.

It might also be useful to add required attributes to the `EventFactory#createEvent()` signature. That way an event processor author cannot forget to add them. But we should avoid making the `#createEvent()` signature too large to ensure it's still okay to handle.

## Tasks

- [ ] tbd

## Open Questions

- [ ] tbd