Function subscribe 

pub async fn subscribe(
    client: &FutuClient,
    securities: &[Security],
    sub_types: &[SubType],
    is_reg_push: bool,
    is_first_push: bool,
) -> Result<()>

订阅行情

订阅指定股票的指定类型行情数据。 订阅后可通过推送接收实时数据更新。