Support for validating message payload

[danielwiehl]

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

Messages sent to a topic endpoint usually contain some kind of payload. In intent-based messaging, data is typically passed in the form of parameters, but it is also possible to pass data in the message body.

So far so good. But the platform does not support out-of-the-box validation of the expected message content. Consequently, receivers have to check the validity of the content themselves.

Describe the solution you'd like

The platform should validate data passed in the message/intent body. Validation should be optional and only performed if a JSON schema is associated with a topic endpoint or capability.

[danielwiehl]

See related issue #145.