The crypto library is missing standards-compliant HKDF (RFC 5869) and an HPKE-compatible ECDH P-256 KEM (RFC…

The crypto library is missing standards-compliant HKDF (RFC 5869) and an HPKE-compatible ECDH P-256 KEM (RFC…: a task in LegoFlow-SWE (Harbor dataset). The library does not currently provide HKDF. Implement HKDF for SHA-256 so that it matches the RFC 5869 test vectors.