Make runners messaging easier.

[richtja]

We want to make the creation of new runners much easy as it is possible. One intricate part of runner development is messaging. Let's make messaging easier for new developers by:

• [x] Adding more/better helper methods on base classes, that will produce the right content.
• [ ] Introducing Python type hints, so that wrong content can be flagged.
• [ ] Introducing some tests doing JSON schema validation.