fill the data field in analytics events

[xvello]

This PR brings payload equivalence with capture.py for payloads sourced from the latest posthog-js version:

• check that the event name is not empty
• add the missing optional timestamp, offset, $set and $set_once toplevel fields
• serialize the RawEvent in the output data field
• set the skip_serializing_if = "Option::is_none" serde annotation where relevant
• make it_matches_django_capture_behaviour unmarshall both data and compare the object instead of the byes
• un-ignore it_matches_django_capture_behaviour as it now passes 🎉
• allow to ommit theRawEvent.properties field in requests, default to an empty dict if not set