Add support for signing/decrypting messages using PKCS#11 tokens

Add a new backend to sequoia-keystore that interacts with PKCS#11 tokens (priority: version 3.2 of the specification). It should implement the Backend, Device, and Key abstractions. These include returning whether a token is available, unlocking tokens, etc.

Support the following algorithms:

• X25519
• Ed25519
• X448
• Ed448
• ML-KEM-786+X25519
• ML-KEM-1024+X448
• ML-DSA-65+Ed25519
• ML-DSA-87+Ed448
• SLH-DSA-SHAKE-128s
• SLH-DSA-SHAKE-128f
• SLH-DSA-SHAKE-256s