Add event.summary as a recommendation for events

[alexvanboxel]

Changes

Events are identified by an event.name and attributes and fields in the body that carry specific meaning. However, since these events will be combined with other logs, event.summary allows a backend to display a human-readable representation of the event.

Merge requirement checklist

• [X] CONTRIBUTING.md guidelines followed.
• [X] Change log entry added, according to the guidelines in When to add a changelog entry.
• If your PR does not need a change log, start the PR title with [chore]
• [ ] schema-next.yaml updated with changes to existing conventions.