Implement `WiFiClass::softAPgetStationNum()`

[felipecrs]

As realized by @jesserockz, these functions are stubbed in:

beken-72xx:

• libretiny/cores/beken-72xx/arduino/libraries/WiFi/WiFiAP.cpp

  Lines 83 to 85 in 9114bc4

  	uint8_t WiFiClass::softAPgetStationNum() {
  	return 0;
  	}

lightning-ln882h:

• libretiny/cores/lightning-ln882h/arduino/libraries/WiFi/WiFiAP.cpp

  Lines 99 to 101 in 9114bc4

  	uint8_t WiFiClass::softAPgetStationNum() {
  	return 0;
  	}

And only implemented in realtek-amb:

libretiny/cores/realtek-amb/arduino/libraries/WiFi/WiFiAP.cpp

Lines 109 to 115 in 9114bc4

	uint8_t WiFiClass::softAPgetStationNum() {
	// the struct is at wifi_conf.c:2576
	client_info_t info;
	info.count = AP_STA_NUM;
	wifi_get_associated_client_list(&info, sizeof(info));
	return info.count;
	}

It would be nice if they were implemented for all boards. This function will be used by:

• [wifi] Use reboot_timeout even with the AP enabled esphome/esphome#9958

[felipecrs]

I think this should work for beken-72xx:

uint8_t WiFiClass::softAPgetStationNum() {
	if (!(getMode() & WIFI_MODE_AP))
		// do not call SDK methods before even initializing WiFi first
		return 0;
	int num = 0;
	if (wlan_ap_sta_num(&num) == 0) {
		return (uint8_t)num;
	}
	return 0;
}

https://github.com/tiancj/bdk_freertos/blob/c2c6bc26faccabdd82ea0d0382c9e86b80313eca/components/wpa_supplicant-2.9/wpa_supplicant/wlan.c#L528-L538

But for some reason it doesn't seem to be working.

I'm testing this with this PR through ESPHome by flashing wirelessly as I don't have serial access to any beken-72xx device.

external_components:
  - source:
      type: git
      url: file:///config/repos/esphome
      ref: reboot_timeout
    components: [wifi]
    refresh: 0s

bk72xx:
  board: generic-bk7231t-qfn32-tuya
  framework:
    version: 0.0.0
    source: /config/repos/libretiny

wifi:
  reboot_timeout: 45s  # <--- this reboot should not happen if a client is connected to AP
  ap:
    ap_timeout: 15s

api:
  reboot_timeout: 0s

Any ideas?