Function generate_x25519_key_pair 

fn generate_x25519_key_pair() -> Result<(HpkePublicKey, HpkePrivateKey), Error>

Generate a X25519 key pair expressed as a raw big-endian fixed-length integer.

We must disambiguate the AsBigEndian trait in-use and this function uses AsBigEndian<Curve25519SeedBin>, which only supports agreement::X25519. For generating P-256, P-384 and P-512 keys see generate_p_curve_key_pair.