fix: CBOR/JSON handling when encoding/deserializing in some code paths

[tmathern]

Changes in this pull request

• Symptom: After the c2pa_cbor update, it became apparent some tests started to fail. The reported error is: AssertionDecoding(could not decode assertion c2pa.time-stamp (version (no version), content type application/json): invalid type: sequence, expected a map). This can happen too in some cases wher c2pa.actions.v2 with test assets from various models providers. We likely want the backwards compatibility here at least to ensure we can parse the data in manifests predating that change.
• There is a default encoding to JSON that seems to be causing the above symptom.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.