Function pk_encrypt 

pub async fn pk_encrypt(
    plain: Vec<u8>,
    public_keys_for_encryption: Vec<SignedPublicKey>,
    private_key_for_signing: SignedSecretKey,
    compress: bool,
    anonymous_recipients: bool,
    seipd_version: SeipdVersion,
) -> Result<String>

Encrypts plain text using public_keys_for_encryption and signs it using private_key_for_signing.