in auto-materialize, handle asset observations with data times but no data versions

[sryza]

Imagine asset graph A->B, where A is an observable source asset and B is an asset with an eager auto-materialize policy.

Currently, if A is observed, the materialize_on_parent_updated rule will consider A updated iff its latest observation has a different data version than its previous observation.

#19830 introduces the ability for observations to have empty data versions, but instead specify a data time.

In these cases, we'd probably want the materialize_on_parent_updated rule to consider A updated iff its latest observation has a different data time than its previous observation.

One way to implement this would be to expand the logic inside _asset_partition_versions_updated_after_cursor.