`events_ingress` : replace our `CloudEvent` model by the Open source `cloudevents/sdk-java` model .

We could be replacing our own implementation of CloudEvent by https://github.com/cloudevents/sdk-java .
I'm still unsure about the added value.... though
Let's see if we have others asking for this..

Start by locating the events_ingress entry point and the repository's current CloudEvent model. Compare that implementation with the cloudevents/sdk-java model and determine whether replacing it has a defined benefit; the issue currently provides no acceptance criteria for what done would mean.

Written by the indexing model from the issue text.