http_server example: WebSocket server to set final flag in async messages

Closes https://github.com/espressif/esp-idf/issues/5405

examples/protocols/http_server/ws_echo_server/README.md

@@ -10,6 +10,7 @@ ws_server_example_test.py could be used as a simple WS client).
 
 The server registers WebSocket handler which echoes back the received WebSocket frame. It also demonstrates
 use of asynchronous send, which is triggered on reception of a certain message.
+Please note that the WebSocket HTTPD server does not automatically fragment messages, so the application code must deal with fragmentation, setting `final` flag as appropriate. Single (complete/unfragmented) WebSocket frames must have the `FIN` flag set to be accepted as valid  (see [RFC6455, section 5.4](https://tools.ietf.org/html/rfc6455#section-5.4)).
 
 ### Hardware Required
 

examples/protocols/http_server/ws_echo_server/main/ws_echo_server.c

@@ -48,6 +48,7 @@ static void ws_async_send(void *arg)
     ws_pkt.payload = (uint8_t*)data;
     ws_pkt.len = strlen(data);
     ws_pkt.type = HTTPD_WS_TYPE_TEXT;
+    ws_pkt.final = true;
 
     httpd_ws_send_frame_async(hd, fd, &ws_pkt);
     free(resp_arg);