AssetCheckSummaryRecord

[dpeng817]

Adds an "AssetCheckState" API; which functions similarly to an AssetRecord. The core conceit is to be able to query just based on asset check key a set of information about the "state" of the asset check (most recent run, for now), without having any references to the event log.

For now, it's actually powered by the event log, however.

How I tested this

• Added a new semi-end-to-end test to our event log storage tests. This required me setting up an instance hooked up to an event log storage; which I think is nice to have anyway.
• There's one failing test here that's confusing me; test_get_event_records_sqlite. I changed it to use the instance passed in by fixture, which should in theory not change the results? But it seems like it has. Probably missing something here.

There will be a follow up cloud pr.