Add Outcomes to Sub-Workflows

It would be really great if sub-workflows could also provide an outcome that can then be used in the calling workflow.

Currently we have some scenarios using subworkflows, where one path leads to cancelling the workflow. In such a case it would be very usefull if the subworkflow could END with a 'Cancelled' outcome, which could then be used to continue to calling workflow appropriately.

Currently we have to work around this situation by setting extra flags on the WorkflowContext in the subworkflow and then checking them in the calling flow.