Function enroll_node 

pub async fn enroll_node(
    server_addr: SocketAddr,
    client_signing_key: &SigningKey,
    member_pubkey: [u8; 32],
    member_sign_pubkey: [u8; 32],
    network_id: [u8; 16],
    credential: EnrollmentCredential,
) -> Result<Cert, String>

Client function to enroll a node with the online threshold CA.