feat: add cas backend tracking to attestations

[Piskoo]

In the attestation process, we want to track which CAS backend was used and whether it was a fallback backend. Add CAS backend information to the attestation in the same format as the existing auth information e.g.

 "cas_backend": {
   "id": "1c879a79-11db-465b-bf3e-da5e338bdaf2"
   "fallback": true
}