Add native AbortSignal support to Firestore Node.js client APIs

# Feature Request: Add native `AbortSignal` support to Firestore Node.js client APIs

I’d like to request **native `AbortSignal` support** in the Node.js Firestore client (`@google-cloud/firestore`), so that operations like `.get()`, `.runQuery()`, `.listDocuments()`, etc., can be cancelled mid-flight.

## Motivation & Use Cases

- Many Firestore operations can run for a significant amount of time (large queries, multi-document reads, long-running streaming operations).
- In modern JavaScript and Node.js, [`AbortController` / `AbortSignal`](https://developer.mozilla.org/docs/Web/API/AbortController) has become the **standard cancellation API** — supported in built-in `fetch()`, many web APIs, and Node.js core libraries since v15.
- In server contexts (e.g., Cloud Functions, HTTP services), it’s often necessary to cancel pending work when the client disconnects or a request times out, to free up resources.

## Technical Feasibility

- Under the hood, `@google-cloud/firestore` calls into [`google-gax`](https://github.com/googleapis/gax-nodejs) and the generated `v1.FirestoreClient`.
- `google-gax` already returns **CancellablePromise** for unary RPCs and **CancellableStream** for streaming RPCs, both of which expose `.cancel()` to abort the gRPC call and tear down the socket.
- Surfacing this at the high-level Firestore API could be done by:
1. Accepting an optional `signal?: AbortSignal` parameter in public methods.
2. Wiring `signal.addEventListener('abort', ...)` to call the underlying `.cancel()` method.

## Example

```js
const controller = new AbortController();

const snapshotPromise = firestore.collection('users').get({ signal: controller.signal });

// Cancel after 1 second
setTimeout(() => controller.abort(new Error('Timeout')), 1000);

await snapshotPromise; // Would reject immediately if aborted
```

## Benefits

- Aligns Firestore client with modern JavaScript ecosystem standards.
- Allows real network-level cancellation instead of “soft” Promise.race cancellation.
- Reduces wasted CPU/memory by aborting in-flight work when no longer needed.

Thanks for considering this request! I’m happy to develop it in a fork myself, help test or provide feedback.