allow dataset alias to add more than one dataset events

[Lee-W]

Why

#34206 (comment)

@task(outlets=[DatasetAlias("example-alias")])
def produce_dataset_events(*, outlet_events):
    outlet_events["example-alias"].add(Dataset("dbt-cloud-table-1"))
    outlet_events["example-alias"].add(Dataset("dbt-cloud-table-2"))

could be confusing if only the second one create a dataset event

What

Save all the datasets when add is invoked.

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[uranusjr]

General idea looks good to me, some notes on implementation.