AIP-82 Save references between assets and triggers

[vincbeck]

Resolves #42510.

This PR adds a new attributes watchers to the Asset class and saves references between assets and triggers in the DB. For example:

trigger = SqsSensorTrigger(sqs_queue="my_queue")
asset = Asset("example_asset_watchers", watchers=[trigger])

with DAG(
    dag_id="example_dataset_watcher",
    schedule=[asset],
    catchup=False,
):
    task = EmptyOperator(task_id="task",)

    chain(task)

This PR creates the trigger in the DB if it does not exist and save the reference between asset and trigger.

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[vincbeck]

@ashb I made this more generic since watchers is also a list and I thought we could just say "if it is a list, then let's convert it to a tuple". Let me know if you have any concerns

[vincbeck]

@Lee-W @uranusjr When working on it I realized that assets are added in the DB from DAG definition but never removed (or at least I did not see the code). Meaning, as a DAG author if I define an asset in my DAG and then later on remove it, the asset is never removed from the DB. Am I wrong? If not, is it intended?