[Observability] emit events to enable external systems to monitor workflow instances

[bvandewe]

What would you like to be added:

(Similarly to the old version of Synapse), it'd be super useful if Synapse could emit asynchronous notifications (e.g. CloudEvents) that can be consumed by external systems in order to monitor workflow instances.

Events could be emitted when the workflow instance was created/completed as well as when any task started/completed.

Why is this needed:

Without these events, it's hard to provide any feedback to end-users about workflow instances.