Reject unsigned init events with data for MID/model stream types

[dav1do]

Description

Currently, an unsigned init event with data will be successfully store and validate. This should be rejected as MID and Model stream types require init events with data are signed. We actually have a test for one of these explicitly, as it was found in the wild (see #404), but it must have been created at a strange time as it would no longer be accepted by js-ceramic.

Technical Information

The event builder should prevent creating these types of events, but the HTTP/recon APIs are currently less strict.

Not entirely sure where this should live, we could enforce it in decode_car, or in the initial (signature/history) validation logic, or in the pipeline (schema validation).