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

まず、現在の platform authenticator の実装と、既存の credential service の境界を確認します。チェックリストを、process API、LSM 統合、WebAuthn 拡張と鍵の種類、および暗号ライブラリの選択を対象とする、個別にスコープを設定した作業に分割します。これらの設計および実装上の問題が解決された場合にのみ、issue は完了となります。