esp_http_client: add support for using certs from global ca store

Closes https://github.com/espressif/esp-idf/issues/3062

components/tcp_transport/include/esp_transport_ssl.h

@@ -40,6 +40,13 @@ esp_transport_handle_t esp_transport_ssl_init();
  */
 void esp_transport_ssl_set_cert_data(esp_transport_handle_t t, const char *data, int len);
 
+/**
+ * @brief      Enable global CA store for SSL connection
+ *
+ * @param      t    ssl transport
+ */
+void esp_transport_ssl_enable_global_ca_store(esp_transport_handle_t t);
+
 /**
  * @brief      Set SSL client certificate data for mutual authentication (as PEM format).
  *             Note that, this function stores the pointer to data, rather than making a copy.