Add validation_state API

[gpeacock]

Add a method to get a validation state from a Reader. We may also want to add this as a field to the report.
The state will be determined as outlined in https://c2pa.org/specifications/specifications/2.1/specs/C2PA_Specification.html#_validation_states

However, we also want to report these states for 1.x even though the spec does not clearly define that.
This is part of a migration strategy. Currently we consider a manifest store valid if the validation_status field is None/Null. But that will no longer be the case for 2.1. So to keep things compatible for old and new, the revised check would be for the validation state.