feat(httpd): add support for asynchronous request handling

This commit adds support for handling multiple requests simultaneously by introducing two new functions: `httpd_req_async_handler_begin()` and `httpd_req_async_handler_complete()`. These functions allow creating an asynchronous copy of a request that can be used on a separate thread and marking the asynchronous request as completed, respectively.

Additionally, a new flag `for_async_req` has been added to the `httpd_sess_t` struct to indicate if a socket is being used for an asynchronous request and should not be purged from the LRU cache.

An example have been added to demonstrate the usage of these new functions.

Closes https://github.com/espressif/esp-idf/issues/10594

Signed-off-by: Harshit Malpani <harshit.malpani@espressif.com>

components/esp_http_server/src/esp_httpd_priv.h

@@ -72,6 +72,7 @@ struct sock_db {
     bool lru_socket;                        /*!< Flag indicating LRU socket */
     char pending_data[PARSER_BLOCK_SIZE];   /*!< Buffer for pending data to be received */
     size_t pending_len;                     /*!< Length of pending data to be received */
+    bool for_async_req;                     /*!< If true, the socket will not be LRU purged */
 #ifdef CONFIG_HTTPD_WS_SUPPORT
     bool ws_handshake_done;                 /*!< True if it has done WebSocket handshake (if this socket is a valid WS) */
     bool ws_close;                          /*!< Set to true to close the socket later (when WS Close frame received) */