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

Comienza revisando la implementación actual del platform authenticator y el límite existente del credential service. Divide la lista de comprobación en trabajos con alcance independiente que cubran la process API, la integración con LSM, las extensiones de WebAuthn y los tipos de clave, y la elección de la biblioteca criptográfica; el issue solo estará terminado cuando se hayan resuelto esas cuestiones de diseño e implementación.