[ENHANCEMENT] Pass Context down when using Visitor for EventSummary

When parsing the EventSummary, we don't pass down the context which could be helpful when shutting down the service to cancel outstanding requests to things like the database.

https://github.com/GoogleChrome/webstatus.dev/blob/b508c57dbf5ccc276826fa623219cbb952faa796/lib/workertypes/types.go#L75-L106

We should pass the context down