Portal restart if desired WiFi network reappears

[mitra42]

Hi Juerd,

I'm trying to deal with what I think could be two common situations.
1: WiFi goes down temporarily
2: Temporary Power cut (very common especially in developing countries) - and WiFi comes back up slower than a device.

In both cases, my concern is unattended devices that with a basic use of WiFiSettings would enter the portal state and get stuck there.

My first attempt to fix this was a simple timeout watchdog, this worked but was annoying if the user is interacting with portal at the time.

I've now got a version that if there is a configured Wifi (i.e. its read a SSID from the SPIFFS/LittleFS) will - after a configurable period (I'm using 2 minutes) - rescan and if it detects the missing WiFi try a restart.

Thanks to all the callbacks I've got it working without changing the library, and am happy to continue using it this way, but I'm thinking it would be a useful addition. I'm not sure what is the best way to integrate it.

My code is at https://github.com/mitra42/frugal-iot/blob/main/system_wifi.cpp#L68 and its stored into WiFiSettings.onPortalWaitLoop at Line 130 based on WiFiSettings.onFailure