Allow named actor of identity assertion to sign endorsements?

[scouten-adobe]

Should we allow non-C2PA signatures, such as the "identity assertion" to also sign endorsements? E.g. a copyright holder wanting to continue to assert their status issues an endorsement to a CDN that is about to transcode their media. The CDN can then be marked as "endorsed" by the copyright holder.

Originally posted by @chalford in #6 (comment)