Figure out whether we can avoid returning Optional for DependentResource.eventSource

[metacosm]

No description provided.

[csviri]

We should mainly consider it because of external resources, and for use cases when the reconciliation is periodically executed - not triggered by infermers.

Let's say the controller manages a set of external resources, one way to implement this is just to add periodic execution for the controller, so it checks all the resource states, and reconciles them. In this case no need for an event source.

[metacosm]

This has been addressed by #949.