Upgrade verification of signed reports to support KEL-backed signatures

[2byrds]

As discussed with @lenkan, we would like to introduce/support KEL-backed signatures verification.
@lenkan suggested it should be an attestation that is chained to the Data Submitter ECR (and maybe eventually another ECR role like Data Signer ECR).
During the discussion we identified the following high level needs:

• Webapp crafts the attestation, given the user ECR
• Extension issues the attestation, chaining it to the ECR
• Webapp adds the KEL-backed signature to the reports.json and adds the CESR stream presentation of the signer to the package
• The user uploads the KEL-backed signed report through the portal and the verifier verifies the submitter, and all KEL-backed signatures/presentations are valid