Add SSE live notifications and Rust SDK subscriber

[digizeph]

Summary

• Add live SSE notifications at GET {root}/events.
• Add an optional Rust SDK SSE subscriber with client-side filtering.
• Update the public documentation for the new API and feature flag.

Changes

• Wire SSE events to newly inserted BrokerItem records in the serve updater process.
• Return 503 from /events when the updater is disabled in the current process.
• Add the sse feature, SseSubscriptionOptions, BrokerItemSubscription, and BgpkitBroker::subscribe_new_files().
• Parse SSE frames in the SDK and filter them client-side by project, collector_id, and data_type.
• Update the changelog and README entries for the endpoint and SDK usage.

Example

curl -N http://127.0.0.1:40064/events

use bgpkit_broker::{BgpkitBroker, SseSubscriptionOptions};
use futures_util::StreamExt;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let broker = BgpkitBroker::new()
        .broker_url("http://127.0.0.1:40064/v3/broker");

    let options = SseSubscriptionOptions::new()
        .project("routeviews")
        .collector_id("route-views2")
        .data_type("updates");

    let mut subscription = broker.subscribe_new_files(options).await?;

    while let Some(item) = subscription.next().await {
        println!("{}", item?.url);
    }

    Ok(())
}