Groupable queries

[reyamir]

Describe the enhancement
Same as NDK https://github.com/nostr-dev-kit/ndk#groupable-queries

Use case
Prevent rate-limited by relays when load user's metadata or other metadata events from difference UI at the same time.

[reyamir]

In case anyone need a example for grouped query for Tauri app, I've made a simple function like this

app.listen_any("request_metadata", move |event| {
    let payload = event.payload();
    let parsed_payload: Payload = serde_json::from_str(payload).expect("Parse failed");
    let handle = handle_clone_child.clone();

    tauri::async_runtime::spawn(async move {
        let state = handle.state::<Nostr>();
        let client = &state.client;

        if let Ok(public_key) = PublicKey::parse(parsed_payload.id) {
            let mut write_queue = state.queue.write().await;
            write_queue.insert(public_key);
        };

        // Wait for [QUEUE_DELAY]
        sleep(Duration::from_millis(QUEUE_DELAY)).await;

        let read_queue = state.queue.read().await;

        if !read_queue.is_empty() {
            let authors: HashSet<PublicKey> = read_queue.iter().copied().collect();

            let filter = Filter::new()
                .authors(authors)
                .kind(Kind::Metadata)
                .limit(200);

            let opts = SubscribeAutoCloseOptions::default()
                .filter(FilterOptions::WaitDurationAfterEOSE(Duration::from_secs(3)));

            // Drop queue, you don't need it at this time anymore
            drop(read_queue);
            // Clear queue
            let mut write_queue = state.queue.write().await;
            write_queue.clear();

            if let Ok(output) = client.subscribe(vec![filter], Some(opts)).await {
                println!("Subscribe success: {:?}", output.success);
            }
        }
    });
});

// Handle notification
...
if let Err(e) = handle_clone.emit("metadata", event.as_json()) {
    println!("Emit error: {}", e)
}
...