Add support for event schemas

[tobyclemson]

Currently events have arbitrary payloads with no typing constraints on their contents. Similarly, event categories can contain arbitrary events. It would be nice to be able to type constrain an event store to only allow category and event shapes.

Whilst such event schemas would assist in ensuring type safety, they would also be useful for use cases such as:

• documentation;
• deprecation and evolution; and
• verification and validation.