Function import_secret_key_pkcs8 

pub async fn import_secret_key_pkcs8(
    variant: EcdhVariant,
    pkcs8: impl Into<Vec<u8>>,
    options: AgreementKeyOptions,
) -> Result<AgreementSecretKey, Error>

Import a static secret key from a PKCS#8 PrivateKeyInfo (DER) whose encoded curve matches the declared variant’s.