Make Datasets hashable

[mpeteuil]

Currently DAGs accept a Collection["Dataset"] as an option for the schedule, but that collection cannot be a set because Datasets are not a hashable type. The interesting thing is that the DatasetModel is actually already hashable, so this introduces a bit of duplication since it's mostly the same implementation. However, Airflow users are primarily interfacing with Dataset, not DatasetModel so I think it makes sense for Dataset to be hashable. I'm not sure how to square the duplication or what __eq__ and __hash__ provide for DatasetModel though.

There was discussion on the original PR that created the Dataset (#24613) about whether to create two classes or one. In that discussion @kaxil mentioned:

I would slightly favour a separate DatasetModel and Dataset so Dataset becomes an extensible class, and DatasetModel just stores the info about the class. So users don't need to care about SQLAlchmey stuff when extending it.

That first PR created the Dataset model as both SQLAlchemy and user space class though. It wasn't until later on (#25727) that the DatasetModel got broken out from Dataset and one became two. That provides a bit of background on why they both exist for anyone reading this who is curious.

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[potiuk]

LGTM. But I would prefer Others to also take a look - maybe there is a good reason for not having it hashable :). And thanks for the thorough research on the past !

BTW. Static checks need fixing on that one (I heartily recommend installing pre-commit and largely stop caring about those failures).