Implement platform authenticator

The current implementation of the platform authenticator demonstrates signing requests with RSA, P-256 and Ed25519 keys
This is a general issue noting some considerations for completing this implementation.

• run the platform authenticator in a separate process from the credential service. Define an API to use between the platform authenticator and the credential service (probably also using D-Bus)
• Investigate integrating with various LSMs (SELinux, AppArmor, Landlock) for increased security of stored credentials.
• Implement the prf, credProp and largeBlob extensions
• Add support for the non-polymorphic versions of the standard WebAuthn COSE key types: -9 for ECDSA w/ P256 curve, and -50 for EdDSA w/ Ed25519 curve (cf. https://github.com/w3c/webauthn/issues/2276)
• Consider whether the ring crate should continue to be used for cryptography functions, or whether to migrate to use OpenSSL or aws-lc-rs

Commencez par examiner l’implémentation actuelle du platform authenticator et la frontière existante du credential service. Divisez la checklist en travaux distincts et délimités couvrant la process API, l’intégration de LSM, les extensions WebAuthn et les types de clés, ainsi que le choix de la bibliothèque cryptographique ; l’issue ne sera terminée que lorsque ces questions de conception et d’implémentation auront été résolues.