Fix CloudEvent model to support extension attributes

[bvandewe]

https://github.com/cloudevents/spec/blob/main/cloudevents/spec.md#extension-context-attributes

"...A CloudEvent MAY include any number of additional context attributes with distinct names, known as "extension attributes". Extension attributes MUST follow the same naming convention and use the same type system as standard attributes. Extension attributes have no defined meaning in this specification, they allow external systems to attach metadata to an event, much like HTTP custom headers."

But the framework currently rejects events like:

{
  "specversion": "1.0",
  "id": "4c668593-18c5-4b96-9ec1-f7d13b2b5ad4",
  "time": "2025-02-03T11:18:03.957400",
  "datacontenttype": "application/json",
  "type": "com.dummy.test.requested.v1",
  "source": "https://dummy-source.com",
  "subject": "some.interesting.concept.key_abcde12345",
  "data": {
    "foo": "bar"
  },
"origin": "gateway-infra",    <<<< EXTENSION ATTRIBUTE
"sequence": 123456,
}

with this error:

INFO:     127.0.0.6:54295 - "POST /events/pub HTTP/1.1" 500 Internal Server Error
2025-02-03 11:17:34,192 ERROR    root:37 An error occured while processing an incoming cloud event: CloudEvent.__init__() got an unexpected keyword argument 'origin'