Store WiFi credentials via preferences

[davidkreidler]

Add the option to load the WiFi credentials on boot. Before this change the user needed to input the WiFi SSID / password every time the device was rebooted. Tested on a ESP32-S3.

[LeonieFierz]

Dear @davidkreidler
Thank you for your contribution!
I had a look at the code and unfortunately we cannot merge it into master. One reason is security concerns with storing WiFi Credentials plain text in the settings.
The other thing is that storing the WiFi credentials should be handled by the caller and not be part of the WiFi Lib Wrapper to keep the Wrapper tidy.
Best, Léonie