Add timestamps to task schema

[stan-dot]

As a user I'd like to see a timestamp for task initalization and task completion so that I can refer back to what scans have I run at which point in time.

The current schema for GET /tasks/{taskId} does not provide that information. that is useful for the #541 history issue

{
  "task_id": "string",
  "task": "string",
  "is_complete": false,
  "is_pending": true,
  "errors": [
    "string"
  ]
}