Axum HTTP tracker: extract Axum extractor for URL path auth key param

[josecelano]

Axum extractor to extract the key path param from the URLs:

Router::new()
    // Announce request
    .route("/announce", get(announce::handle_without_key).with_state(tracker.clone()))
    .route("/announce/:key", get(announce::handle_with_key).with_state(tracker.clone()))
    // Scrape request
    .route("/scrape", get(scrape::handle_without_key).with_state(tracker.clone()))
    .route("/scrape/:key", get(scrape::handle_with_key).with_state(tracker.clone()))
    // Add extension to get the client IP from the connection info
    .layer(SecureClientIpSource::ConnectInfo.into_extension())

It extracts the :key param. I'm using an extractor because we have a custom error response, and the handlers are much cleaner this way and we remove the duplicate code.