http_server: adds WebSocket support

This commit adds the WebSocket support for esp_http_server
library. It mainly does:

- Handling WebSocket handshake
  - Parsing HTTP upgrade request
  - Reply the upgrade request
- Receive WebSocket packets
  - Parse header, decode to a struct
  - Unmask payload (if required)
- Send WebSocket frames
- Receive WebSocket frame
- Automatic control frame handling

Merges https://github.com/espressif/esp-idf/pull/4306
Closes https://github.com/espressif/esp-idf/issues/4819

components/esp_http_server/src/httpd_sess.c

@@ -281,7 +281,9 @@ bool httpd_sess_pending(struct httpd_data *hd, int fd)
     if (sd->pending_fn) {
         // test if there's any data to be read (besides read() function, which is handled by select() in the main httpd loop)
         // this should check e.g. for the SSL data buffer
-        if (sd->pending_fn(hd, fd) > 0) return true;
+        if (sd->pending_fn(hd, fd) > 0) {
+            return true;
+        }
     }
 
     return (sd->pending_len != 0);