我说的底层api是诸如esp_mqtt_client_subscribe，请用ESP32-S2的底层库来写

好的，以下是使用ESP32-S2底层库来订阅MQTT主题的示例代码：

#include <stdio.h>
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "esp_log.h"
#include "esp_wifi.h"
#include "esp_event.h"
#include "esp_mqtt.h"

#define WIFI_SSID "your_wifi_ssid"
#define WIFI_PASS "your_wifi_password"
#define MQTT_BROKER_URL "mqtt://test.mosquitto.org"
#define MQTT_TOPIC "test/topic"

static const char *TAG = "MQTT_EXAMPLE";

static esp_err_t wifi_event_handler(void *ctx, system_event_t *event)
{
    switch (event->event_id) {
        case SYSTEM_EVENT_STA_START:
            ESP_LOGI(TAG, "Wi-Fi client started");
            esp_wifi_connect();
            break;
        case SYSTEM_EVENT_STA_CONNECTED:
            ESP_LOGI(TAG, "Connected to AP");
            break;
        case SYSTEM_EVENT_STA_DISCONNECTED:
            ESP_LOGI(TAG, "Disconnected from AP");
            esp_wifi_connect();
            break;
        default:
            break;
    }
    return ESP_OK;
}

static esp_err_t mqtt_event_handler(esp_mqtt_event_handle_t event)
{
    switch (event->event_id) {
        case MQTT_EVENT_CONNECTED:
            ESP_LOGI(TAG, "MQTT client connected");
            esp_mqtt_client_subscribe(event->client, MQTT_TOPIC, 0);
            break;
        case MQTT_EVENT_DISCONNECTED:
            ESP_LOGI(TAG, "MQTT client disconnected");
            break;
        case MQTT_EVENT_SUBSCRIBED:
            ESP_LOGI(TAG, "MQTT client subscribed to topic '%s'", MQTT_TOPIC);
            break;
        case MQTT_EVENT_DATA:
            ESP_LOGI(TAG, "MQTT client received data");
            // Handle received data here
            break;
        default:
            break;
    }
    return ESP_OK;
}

void app_main()
{
    ESP_ERROR_CHECK(esp_netif_init());
    ESP_ERROR_CHECK(esp_event_loop_create_default());
    esp_netif_create_default_wifi_sta();

    wifi_init_config_t wifi_init_cfg = WIFI_INIT_CONFIG_DEFAULT();
    ESP_ERROR_CHECK(esp_wifi_init(&wifi_init_cfg));

    wifi_config_t wifi_cfg = {
        .sta = {
            .ssid = WIFI_SSID,
            .password = WIFI_PASS,
        },
    };
    ESP_ERROR_CHECK(esp_wifi_set_mode(WIFI_MODE_STA));
    ESP_ERROR_CHECK(esp_wifi_set_config(ESP_IF_WIFI_STA, &wifi_cfg));
    ESP_ERROR_CHECK(esp_wifi_start());

    esp_mqtt_client_config_t mqtt_cfg = {
        .uri = MQTT_BROKER_URL,
        .event_handle = mqtt_event_handler,
    };
    esp_mqtt_client_handle_t mqtt_client = esp_mqtt_client_init(&mqtt_cfg);
    ESP_ERROR_CHECK(esp_mqtt_client_start(mqtt_client));
}

这段代码使用ESP32-S2的底层库实现了Wi-Fi连接和MQTT订阅。在`app_main`函数中，我们首先初始化了Wi-Fi，并且设置了Wi-Fi连接的SSID和密码。然后，我们初始化了MQTT客户端，并且设置了MQTT服务器的URL和事件回调函数。在事件回调函数中，我们可以处理MQTT客户端的连接、断开连接、订阅和接收数据等事件。在`mqtt_event_handler`函数中，我们通过调用`esp_mqtt_client_subscribe`函数来订阅了一个MQTT主题。当我们成功订阅了主题后，就可以在`MQTT_EVENT_DATA`事件中处理接收到的数据了。