feat: Add custom transport configuration

Today there is no way to add a new transport without applying
modifications to the transport list. This impose limitations on the
client usage. Adding the custom configuration we enable user defined
transports.

lib/include/mqtt_client_priv.h

@@ -91,6 +91,7 @@ typedef struct {
     bool use_secure_element;
     void *ds_data;
     int message_retransmit_timeout;
+    esp_transport_handle_t transport;
 } mqtt_config_storage_t;
 
 typedef enum {