Allow manual creation of additional NSubprocess while (parent) task is not yet DONE

[emihir0]

As described here: https://stackoverflow.com/questions/59934735/dynamic-nsubprocess-start-additional-subprocess-after-nsuprocess-is-started

It would be useful to have additional behaviour in NSubprocess such that we can start up additional ones while the task is still alive (in STARTED state).

Right now NSubprocess is started up with items that are calculated when the (parent) task is started. The problem with this approach is that there is, sometimes, a need to be able to add additional items to the task.

For example a customer makes an order through phone, we submit the order, each order item/line is an item in the nsubprocess task (ie. a process is fired up to fulfill the order item). Then they call 5 minutes later and want to add additional item - this is very difficult. Current options are to either roll back all the way and cancel the task itself, or to cancel the full order process and re-try all the data.

It would be useful to be able to start up additional subprocesses when the task is not yet DONE, eg something like:

    @Activation.status.transition(source=STATUS.STARTED)
    def start_subprocess(self, item):
        self.flow_task.subflow_task.run(self.task, item)