Use parameter annotation for designating external events

[alexander-yevsyukov]

Currently in order to react on an external event we need to set the external attribute of the React and Command annotations to true. Although it works, it's a bit confusing especially when it comes to commanding methods:

@Command(external = true)
RemoveTaskFromProject movingPolicy(TaskMoved event) {
    ...
}

It would be more natural if we annotate the parameter instead:

@Command
RemoveTaskFromProject movingPolicy(@External TaskMoved event) {
    ...
}

Event though we can probably try to determine the external nature of the event by its class via analysing @BoundedContext annotations, we do want to have this fact declared directly in the code of the application. It would ease the analysis of context mapping.

[alexander-yevsyukov]

@armiol, this is the issue on external events that we've discussed vocally.