ESPAsyncWebserver handler example with String query argument

This example is based on our basic example and shows how to use an String query parameter, e.g. http://192.168.1.112/api/test?param=abc123

server.on("/api/test", HTTP_GET, [](AsyncWebServerRequest *request) {
      String param = request->getParam("param")->value();
      // TODO: Do something with param!

      // Respond with JSON {"status": "ok"}
      AsyncResponseStream *response = request->beginResponseStream("application/json");
      DynamicJsonDocument json(1024);
      json["status"] = "ok";
      json["param"] = param;
      serializeJson(json, *response);
      request->send(response);
});