allow for a meta-chainhook that informs users when a predicate status changes

[rafaelcr]

It would be useful for clients to be informed when their predicate has changed status (expired, completed, failed, etc.) so they can perform actions based on this event. Right now we don't have a way to do that other than having clients call /v1/chainhook/:uuid directly looking for status changes.

[aulneau]

🙏