ESP32: Improve WiFi Configuration CGI

[tidklaas]

• Move ESP32 functionality into esp32_wifi.c.
• Remove wifiScanDoneCb(), add initCgiWifi() and cgiWifiEventCb(). Main application needs to call initCgiWifi() before making any calls to the esp32_wifi module. All system events should be forwarded from the main system event handler to cgiWifiEventCb().
• Manage all configuration changes via FreeRTOS timer callbacks. Timer task's stack size might have to be increased.