Feature Request: Operator Defined Activation Lifecycling Events

Problem:

Our openwhisk operation is strictly through a management orchestration layer built on top of openwhisk. We strictly use non-blocking activations, which requires us to get notification of when the activation completes. Otherwise we are stuck with polling the api to find out if the activation to see if the activation has completed, which doesn't scale. So our current implementation listens to the `events` topic to get notice that the activation has finished. We started out listening to the completed topics, but that no longer has all of the information we need so we listen to events now.

The problem with this approach is that 1. the events topic produces lots of metrics that we just eat for this consumer and more importantly 2. it's a potential single bottleneck across all users / functions of the platform.

Potential Solution:

It would be nice if the invocation request could include some metadata about where to produce notice that the activation has completed. Or maybe it could be an annotation on the function itself. So each activation would have information on what kafka to produce the result to. Since we already produce the activation completion event to events, I don't think this should be too hard to accomplish.

For us at least, the way I envision it is each namespace would have a kafka topic to produce to to keep namespaces from potentially impacting each other. If it were to go that route, the user may not have to supply where to send the activation result and openwhisk just creates an activation topic for each namespace to produce completion acks to?

I see this as being beneficial to anyone that leverages non-blocking activations and doesn't want to rely on a single topic for activation event notifications for the entire platform.