Implement validation of redactions #239
Description
We don't currently have any example files for assertion redaction (does c2patool support creating them?). Once we do, we can implement and test the validation as described in the C2PA specification. This includes status codes assertion.notRedacted, assertion.selfRedacted, and others.
Special attention must be given to the (valid) case where a redacted assertion is still present but its data has been replaced with zero bytes.