Allow namespacing of ArgoEvent when published from a step

[ob-uk]

Currently the events that @trigger_on_finish depends on are namespaced with a scheme such as project.branch.flow.Event. This is neat because when I want to specify a dependency between flows, I can separate them by project/branch so that a flow on a particular branch is triggered by an upstream flow also in that branch.

This functionality does not exist for the ArgoEvent object (and @trigger). Therefore if someone has some conditional triggering logic such as:

@step
def my_logic(self): 
   if this_condition_true: 
       ArgoEvent('condition_true').publish()
   else: 
       ArgoEvent('condition_false').publish() 
   self.next(self.end)

Then they'd have to have custom logic to construct the event name based on the project and branch. At the same time, the downstream flow's @trigger would need to have specific event names constructed by the user to make sure they're only triggered by events in that namespace.

It would be great to introduce options like ArgoEvent(..., namespaced = True) and @trigger(..., namespaced = True) so that the users don't need to know how Metaflow internally constructs events and they can get the guarantee of namespace level isolation.

For users using this functionality from an external system (not a Metaflow flow), they can disable namespacing.