Rely on Kubernetes resource refs in destinations

[pantierra]

No description provided.

[mneagul]

The current implementation complicates the whole interaction with KNative.

What is important: K_NATIVE is an URL which can receive a POST event of a CloudEvent.

The K_CE_OVERRIDES specified an JSON encoded environment variables representing key/values that should be overwritten by the code. The Python code should ignore this as much as possible and apply it before triggering the POST corresponding to the generated event.

[mneagul]

What I'm expecting:

• The code in eoAPI only expects to have K_SINK defined in an environment variable
• The ref vs url logic should be outside the code, and should only reside in the helm chart
• The Helm chart, based on the configuration, should set K_SINK only is url is set, otherwise it should create a SinkBinding which should match the deployment/stefullset and inject the environment variable. This last part is the responsibility of KNative and no logic, besides the expectation on the presence ok K_SINK, should reside in the application code.

[mneagul]

The Python side of code should only care of K_SINK. If it's set then it has to POST to that endpoint.
A special attention should be given to the env K_CE_OVERRIDES. This is a JSON object that specifies overrides to the outbound event.

See this:

• https://knative.dev/docs/eventing/custom-event-source/sinkbinding/

[pantierra]

Thanks for your reviews and support @mneagul!

Closing this PR here in favour of these follow-ups:

• Start to support SinkBinding in helm-chart. #2
• Add K_CE_OVERRIDE to cloudevents plugin. #3