mqtt_config: Add config value to disable keepalive mechanism

Added a separate config value to allow disable the keepalive mechanism (and keep the existing API the same)
Internally though, the keepalive value (in connect_info) is in line with 3.1.2.10 Keep Alive from mqtt spec:
* keepalive=0: Keep alive mechanism disabled (server not to disconnect the client on its inactivity)
* period in seconds to send a Control packet if inactive

Closes https://github.com/espressif/esp-mqtt/issues/179

include/mqtt_client.h

@@ -199,6 +199,7 @@ typedef struct {
     bool use_secure_element;                /*!< enable secure element for enabling SSL connection */
     void *ds_data;                          /*!< carrier of handle for digital signature parameters */
     int network_timeout_ms;                 /*!< Abort network operation if it is not completed after this value, in milliseconds (defaults to 10s) */
+    bool disable_keepalive;                 /*!< Set disable_keepalive=true to turn off keep-alive mechanism, false by default (keepalive is active by default). Note: setting the config value `keepalive` to `0` doesn't disable keepalive feature, but uses a default keepalive period */
 } esp_mqtt_client_config_t;
 
 /**