AIP-82 Handle trigger serialization

[vincbeck]

Handle serialization of triggers in DAGs as part of AIP-82. With AIP-82 (External event driven scheduling in Airflow) you can define DAGs as below:

trigger = FileTrigger(....)
asset = Asset("<my_queue>", watchers=[AssetWatcher(name="my_file_watcher", trigger=trigger)])
 
with DAG(
    dag_id=DAG_ID,
    schedule=asset,
):
    empty_task = EmptyOperator(task_id="empty_task")
 
    chain(empty_task)

Triggers can be instantiated as part of a DAG. As such, we need to serialize them.

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[vincbeck]

I addressed your comments but I also added something new. It was actually a suggestion from @dstandish that I think is a great idea! I introduced the concept of AssetWatcher that is a thin wrapper around trigger. Explanations:

Instead of having:

trigger = FileTrigger(....)
asset = Asset("<my_queue>", watchers=[trigger])

Now it is:

trigger = FileTrigger(....)
asset = Asset("<my_queue>", watchers=[AssetWatcher(name="my_queue_watcher", trigger=trigger)])

AssetWatcher does not add much besides just a name, but I think it is important because, in the future, we might want to surface the relations between assets and triggers in the graph view in the UI. Representing a trigger in the graph can be tricky because triggers have 2 pieces of information: classpath and kwargs. None of them are very suitable for displaying purposes. classpath is debatable but it is anyway not representable of the trigger. Example: you might have an asset with 2 watchers: each one uses the trigger FileTrigger to monitor a different file. Having only the classpath displayed will not help the user to understand which one is which. Hence the name.

[vincbeck]

@uranusjr @ashb what do you guys think?

[ashb]

I'll take another look today or tomorrow.

[ashb]

Looks much better thanks, but a few changes I'd like you to make please.

[vincbeck]

Other questions/concerns? :)

[vincbeck]

@ashb Are you okay with these changes? I'll make the changes related to the discussion we had on Slack (creating the interface BaseEventTrigger) in a separate PR.

[ashb]

All good, thanks @vincbeck