ws_client: added subprotocol configuration option to websocket client

closes https://github.com/espressif/esp-idf/issues/3893

components/esp_websocket_client/include/esp_websocket_client.h

@@ -91,6 +91,7 @@ typedef struct {
     int                         buffer_size;                /*!< Websocket buffer size */
     const char                  *cert_pem;                  /*!< SSL Certification, PEM format as string, if the client requires to verify server */
     esp_websocket_transport_t   transport;                  /*!< Websocket transport type, see `esp_websocket_transport_t */
+    char                        *subprotocol;               /*!< Websocket subprotocol */
 } esp_websocket_client_config_t;
 
 /**