Add support for Knative Broker/Trigger

[nicolaferraro]

Latest Knative eventing adds new objects: broker and trigger.

The base idea is to make a "event mesh" where there are sources that publish events/messages to the broker and services can be "triggered" when any event with specific features (filters) is published.

This is a higher level thing on top of channels/subscriptions.

The standard architecture is made of a single broker per namespace named 'default' (even if multiple brokers can be created if needed).

We should find define a knative:broker endpoint that can be used both as consumer and producer.

[nicolaferraro]

Fixed by #1012