Allow to define operations inline

[danielmitterdorfer]

At the moment, users can specify operations only in a dedicated operations block but for simple tracks it would be more convenient if we could just specify operations "inline" when specifying tasks in the schedule element of a challenge.

We should also simplify the syntax a bit. For operations that do not take any parameters, we could just refer to their name instead of specifying an object, i.e. the following two definitions should be equivalent:

{
  "schedule": [
    {
      "operation": {
        "name": "force-merge",
        "operation-type": "force-merge"
      }
    }
  ]
}

{
  "schedule": [
    {
      "operation": "force-merge"
    }
  ]
}

Note that in these two cases an operation is defined inline with its corresponding task and not just referenced (i.e. there is no corresponding definition in the operations block).