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