[Python/CloudEvents] JobSink Integration / Long Running Functions

[pierDipi]

As described in #2586, we need to integrate the Python CloudEvents functions with JobSink

• when the env variable K_EXECUTION_MODE is equal to batch the python runtime will read the event from the file and pass it to the handler
• If the function returns a new event it would be produced to an endpoint that is pointed by K_SINK env variable (if present) (so that you can use JobSink in combination with SinkBinding to have "job callbacks")

[pierDipi]

For this one, we will need to update the new python library: https://github.com/knative-extensions/func-python