Add support for field selector for onKubernetesEvent hooks

[dmgtn]

Example, that seems to be self-explaining:

{
  "onKubernetesEvent": [
    {
      "fieldsSelector": {
        "matchExpressions":[
          {
            "field": "status.phase",
            "operator": "Equal",           # Equal or NotEqual
            "value": "Pending", 
          }, 
        ]
      }
    },
    ...
  ]
}

[diafour]

edit:

• selector → fieldSelector
• matchFieldExpressions → matchExpressions

[diafour]

Implemented in #47