attest: Add signal that Data section in an event corresponds to the event digest

Use case:
- Sometimes interesting information is put in the Data section of an event
- In these cases the event is such that the digest extended matches the digest of the Data section
- In this case, the consumer of the Event would like to use the contents of the data section _if and only if_ the data section matches the digest of the event

Proposal:
- Add a method/field that would indicate whether the data field of an `Event` is verified by the digest or not.