Consider moving Knative API to duck types

[squakez]

Requirement

We're making usage of Knative stable API but we depends on knative serving/eventing dependencies which are changing frequently. We should analyze the possibility to move to Knative API duck types instead, as done for Keda and Strimzi, for example. If we only depends on the API as I suspect, in the long run it will be reduce the maintenance burden.