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=[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.

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.