rcgen 0.14 replaced the separate Certificate + KeyPair args with a
unified Issuer type. Migrates ensure_ca and generate_service_cert:
- Load path: Issuer::from_ca_cert_der replaces the old
CertificateParams::from_ca_cert_pem + self_signed round-trip.
- Generate path: Issuer::new(params, key_pair) constructs directly
from the params used for self_signed (no DER re-parse).
- signed_by takes (&key_pair, &issuer) instead of (&key_pair, &cert, &key).
Also drops thiserror v1 from the dep tree (rcgen 0.14 uses v2).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
