Add task iteration on Task Context for repeteable tasks

[fjtirado]

For this workflow

document:
  dsl: 1.0.0-alpha5
  namespace: example
  name: SwitchLoopWait
  version: 0.1.0
do:
  - inc:
     set:
        count: ${.count+1}
     then: looping
  - looping:
       switch:
         - loopCount:
             when: .count < ($workflow.input.maxCount // 10000)
             then: waitABit
         - default:
             then: exit
  - waitABit:
       wait:
         milliseconds: 1000
       then: inc

The looping task is repeteable, meaning that task events for the same position and status combination might be triggered more than once.
It would be nice to be able to distinguish different executions of that task. One possible solution is to add an iteration property to TaskContext