feat(lexicon): add app.certified.signature.* lexicons for cryptographic attestations

Add three new lexicons for signature/attestation support:
- app.certified.signature.inline: Inline signature object with JOSE algorithm identifiers
- app.certified.signature.list: Container with signatures array (open union)
- app.certified.signature.proof: Remote attestation proof record with CID

Author: satyam-mishra-pce