Implement event dispatching webhook

[stefanprodan]

This PR adds a a new webhook type named event. When an event webhook is present in the canary spec, Flagger will dispatch all events to that URL or URLs. This allows Flagger users to implement a webhook receiver that posts events to Slack, MS Teams or any other platform.

The event receiver should expose a URL that accepts HTTP POST with the following JSON body:

{
  "name": "string (canary name)",
  "namespace": "string (canary namespace)",
  "phase": "string (canary phase)",
  "metadata": {
    "eventMessage": "string (canary event message)",
    "eventType": "string (canary event type)",
    "timestamp": "string (unix timestamp ms)"
  }
}

The receiver can create alerts based on the received phase (possible values: Initialized, Waiting, Progressing, Promoting, Finalising, Succeeded or Failed).

Fix: #393
Fix: #242
Fix: #128