Received events should be logged in human-readable form

[cardil]

After the event is received by the showcase app, the event should be logged to the console in human-readable form. Best would be to log the event in the same way the Knative's event-display image does it:

☁️  cloudevents.Event
Validation: valid
Context Attributes,
  specversion: 1.0
  type: samples.http.mode3
  source: /apis/v1/namespaces/default/pingsources/ping-source
  id: e8fa7906-ab62-4e61-9c13-a9406e2130a9
  time: 2020-03-02T20:52:00.0004957Z
  datacontenttype: application/json
Data,
  {
    "id": 0,
    "message": "Hello world"
  }