[pubsub] Cancellable MessageStream

[yoshidan]

Added MessageStream#cancellable() to finish subscribe messages gracefully.

https://github.com/yoshidan/google-cloud-rust/issues/266

use std::time::Duration;
use futures_util::StreamExt;
use google_cloud_pubsub::client::{Client, ClientConfig};
use google_cloud_googleapis::pubsub::v1::PubsubMessage;
use google_cloud_pubsub::subscription::{SubscribeConfig, SubscriptionConfig};
use google_cloud_gax::grpc::Status;

async fn run(config: ClientConfig) -> Result<(), Status> {
     // Creating Client, Topic and Subscription...
     let client = Client::new(config).await.unwrap();
     let subscription = client.subscription("test-subscription");

     // Read the messages as a stream
     let mut stream = subscription.subscribe(None).await.unwrap();
     let cancellable = stream.cancellable();
     let task = tokio::spawn(async move {
         // None if the stream is cancelled
         while let Some(message) = stream.next().await {
             message.ack().await.unwrap();
         }
     });
     tokio::time::sleep(Duration::from_secs(60)).await;
     cancellable.cancel();
     let _ = task.await;
     Ok(())
}