wifi: optimize scan feature

1. add the threshold of match AP in fast scan
2. add blacklist feature
3. make sure would wrong if password didn't set
4. add scan example

components/esp32/include/esp_wifi_types.h

@@ -162,6 +162,11 @@ typedef enum {
     WIFI_CONNECT_AP_BY_SECURITY,          /**< Sort match AP in scan list by security mode */
 }wifi_sort_method_t;
 
+typedef struct {
+    int8_t              rssi;                      /**< The minimum rssi to accept in the fast scan mode */
+    wifi_auth_mode_t    authmode;                  /**< The weakest authmode to accept in the fast scan mode */
+}wifi_fast_scan_threshold_t;
+
 typedef enum {
     WIFI_PS_NONE,    /**< No power save */
     WIFI_PS_MODEM,   /**< Modem power save */
@@ -196,6 +201,7 @@ typedef struct {
     uint8_t bssid[6];     /**< MAC address of target AP*/
     uint8_t channel;       /**< channel of target AP. Set to 1~13 to scan starting from the specified channel before connecting to AP. If the channel of AP is unknown, set it to 0.*/
     wifi_sort_method_t sort_method;    /**< sort the connect AP in the list by rssi or security mode */
+    wifi_fast_scan_threshold_t  threshold;     /**< When scan_method is set to WIFI_FAST_SCAN, only APs which have an auth mode that is more secure than the selected auth mode and a signal stronger than the minimum RSSI will be used. */
 } wifi_sta_config_t;
 
 typedef union {