[pubsub] AbortSignal support for the subscriber client

### A screenshot that you have tested with "Try this API".

This problem applies specifically to the Node.js client. It's not an issue with the API itself.

### What would you like to see in the library?

A way to abort cleanly when the service needs to exit (without setting a very low timeout). It would be nice if the pull method supported AbortSignal like so since AbortSignal is becoming the standard way to abort promises:

```ts
const abortController = new AbortController();
const client = new v1.SubscriberClient({ projectId });

const [response] = await client.pull({
subscription: subscriptionId,
maxMessages: 100,
}, {
signal: abortController.signal <-- This
});
```

### Describe alternatives you've considered

_No response_

### Additional context/notes

_No response_