json-schema is more flexible and allow us to specify object with unknown attributes. 

This would allow us to lift the restriction of sending the payload and context as json. Additionaly, json-schema supports optional fields, assigning defaults and optional fields would decrease the barrier in apps having to specify fields that are not really used.

It could be a good opportunity to start migrating to the org_id (having the field as optional).

Eventually we are going to adopt an schema for the eventing work, and json-schema is also proposed there. This task moves towards that goal.