Change CRD duration type to date

Similar to the Age column, also the Duration could be stored as type: date with start timestamp so we wouldn't need to compute the duration by our own:

    additionalPrinterColumns:
    - name: STATUS
      type: string
      description: State of the execution
      jsonPath: .status.executionState
    - name: Duration
      type: string
      description: Duration of the execution
      jsonPath: .status.executionDuration
    - name: Age
      type: date
      jsonPath: .metadata.creationTimestamp