val progressTracker: ProgressTracker
Override this to provide a ProgressTracker. If one is provided and stepped, the framework will do something helpful with the progress reports e.g record to the audit service. If this flow is invoked as a subflow of another, then the tracker will be made a child of the current step in the parent. If it's null, this flow doesn't track progress.
Note that this has to return a tracker before the flow is invoked. You can't change your mind half way through.