chore: Implement Sync for Consumer

[cirias]

Hi! This request adds the auto implementation of Sync for Consumer.

Currently, Sync is not implemented for Consumer, which causes error when trying to move the consumer to a backgound task. See the code below as an example.

async main() {
    let consumer: Consumer<TestData, _> = pulsar
        .consumer()
        .build()
        .await?;

    let mut app = App { consumer };

    tokio::task::spawn(app.run()).await.unwrap();
}

struct App {
    consumer: Consumer<TestData, TokioExecutor>,
}

impl App {
    async fn run(&mut self) {
        while let Some(_msg) = self.consumer.try_next().await.unwrap() {
            self.dosomething().await;
        }
    }

    async fn dosomething(&self) {}
}

---

• Updated the example.

[tisonkun]

Thank you!