[FEATURE REQUEST] Callback URL for Convoy's events.

[subomi]

Describe Feature Convoy as a service should be able to publish webhook events about its webhook events.

Why? One use-case is when implementing the app portal, especially for users who have a dashboard already, not everyone wants to always have to call Convoy's API. They'd prefer to get notified about certain entities and show their users. Entities like:

• Subscriptions Status
• EventDelivery Status