Function group_encrypt 

pub async fn group_encrypt<R>(
    sender_key_store: &mut dyn SenderKeyStore,
    sender: &ProtocolAddress,
    distribution_id: Uuid,
    plaintext: &[u8],
    csprng: &mut R,
) -> Result<SenderKeyMessage, SignalProtocolError>
where
    R: Rng + CryptoRng,