experimental/mqtt_cxx: Adds a C++ Mqtt client wrapper

- Base class with separated event handlers for each Mqtt client event.
- Topic matcher added to support data events.
- Filter class to allow only mqtt valid topic filters.
- Initial code for unit test on the host.

examples/cxx/experimental/esp_mqtt_cxx/tcp/main/CMakeLists.txt

@@ -0,0 +1,3 @@
+idf_component_register(SRCS "mqtt_tcp_example.cpp"
+                    INCLUDE_DIRS ".")
+target_compile_options(${COMPONENT_LIB} PRIVATE "-std=gnu++17")

examples/cxx/experimental/esp_mqtt_cxx/tcp/main/Kconfig.projbuild

@@ -0,0 +1,9 @@
+menu "Example Configuration"
+
+    config BROKER_URL
+        string "Broker URL"
+        default "mqtt://mqtt.eclipse.org"
+        help
+            URL of the broker to connect to
+
+endmenu

examples/cxx/experimental/esp_mqtt_cxx/tcp/main/mqtt_tcp_example.cpp

@@ -0,0 +1,81 @@
+/* C++ MQTT (over TCP) Example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+
+#include "nvs_flash.h"
+#include "protocol_examples_common.h"
+
+#include "esp_log.h"
+#include "esp_mqtt.hpp"
+#include "esp_mqtt_client_config.hpp"
+
+namespace mqtt = idf::mqtt;
+
+namespace {
+constexpr auto *TAG = "MQTT_EXAMPLE";
+
+class MyClient final : public mqtt::Client {
+public:
+    using mqtt::Client::Client;
+
+private:
+    void on_connected(esp_mqtt_event_handle_t const event) override
+    {
+        using mqtt::QoS;
+        subscribe(messages.get());
+        subscribe(sent_load.get(), QoS::AtMostOnce);
+    }
+    void on_data(esp_mqtt_event_handle_t const event) override
+    {
+        if (messages.match(event->topic, event->topic_len)) {
+            ESP_LOGI(TAG, "Received in the messages topic");
+        }
+    }
+    mqtt::Filter messages{"$SYS/broker/messages/received"};
+    mqtt::Filter sent_load{"$SYS/broker/load/+/sent"};
+};
+}
+
+extern "C" void app_main(void)
+{
+    ESP_LOGI(TAG, "[APP] Startup..");
+    ESP_LOGI(TAG, "[APP] Free memory: %d bytes", esp_get_free_heap_size());
+    ESP_LOGI(TAG, "[APP] IDF version: %s", esp_get_idf_version());
+
+    esp_log_level_set("*", ESP_LOG_INFO);
+    esp_log_level_set("MQTT_CLIENT", ESP_LOG_VERBOSE);
+    esp_log_level_set("MQTT_EXAMPLE", ESP_LOG_VERBOSE);
+    esp_log_level_set("TRANSPORT_TCP", ESP_LOG_VERBOSE);
+    esp_log_level_set("TRANSPORT_SSL", ESP_LOG_VERBOSE);
+    esp_log_level_set("TRANSPORT", ESP_LOG_VERBOSE);
+    esp_log_level_set("OUTBOX", ESP_LOG_VERBOSE);
+
+    ESP_ERROR_CHECK(nvs_flash_init());
+    ESP_ERROR_CHECK(esp_netif_init());
+    ESP_ERROR_CHECK(esp_event_loop_create_default());
+
+    /* This helper function configures Wi-Fi or Ethernet, as selected in menuconfig.
+     * Read "Establishing Wi-Fi or Ethernet Connection" section in
+     * examples/protocols/README.md for more information about this function.
+     */
+    ESP_ERROR_CHECK(example_connect());
+
+    mqtt::BrokerConfiguration broker{
+        .address = {mqtt::URI{std::string{CONFIG_BROKER_URL}}},
+        .security =  mqtt::Insecure{}
+    };
+    mqtt::ClientCredentials credentials{};
+    mqtt::Configuration config{};
+
+    MyClient client{broker, credentials, config};
+
+    while (true) {
+        constexpr TickType_t xDelay = 500 / portTICK_PERIOD_MS;
+        vTaskDelay(xDelay);
+    }
+}