esp_http_server: support dynamic payload len for ws server

Closes https://github.com/espressif/esp-idf/issues/6433

components/esp_http_server/src/esp_httpd_priv.h

@@ -102,6 +102,7 @@ struct httpd_req_aux {
     bool ws_handshake_detect;                       /*!< WebSocket handshake detection flag */
     httpd_ws_type_t ws_type;                        /*!< WebSocket frame type */
     bool ws_final;                                  /*!< WebSocket FIN bit (final frame or not) */
+    uint8_t mask_key[4];                            /*!< WebSocket mask key for this payload */
 #endif
 };