Add PBKDF2 key derivation with HMAC‑SHA‑256 to Soter

Add PBKDF2 key derivation with HMAC‑SHA‑256 to Soter: a task in LegoFlow-SWE (Harbor dataset). We need a public key derivation function in the Soter library that implements PBKDF2 with HMAC‑SHA‑256 (RFC 8018). This will allow deriving encryption keys from passphrases.