[WiFi] Cleanup code, split ESP32/ESP8266 specific

[TD-er]

Finally going to clean up the WiFi code and restructure it.
Hopefully this will also show where the long lasting WiFi reconnect issues may be...

Fixes: #3870

[TD-er]

Notes/bookmarks:

AP API: https://github.com/espressif/arduino-esp32/blob/master/libraries/WiFi/src/WiFiAP.h#L44-L57
STA API: https://github.com/espressif/arduino-esp32/blob/master/libraries/WiFi/src/WiFiSTA.h#L49-L83
ETH API: https://github.com/espressif/arduino-esp32/blob/master/libraries/Ethernet/src/ETH.h#L125-L163
PPP API: https://github.com/espressif/arduino-esp32/blob/master/libraries/PPP/src/PPP.h#L26-L80
Common for every interface: https://github.com/espressif/arduino-esp32/blob/master/libraries/Network/src/NetworkInterface.h#L39-L76

Manager api:
Main: https://github.com/espressif/arduino-esp32/blob/master/libraries/Network/src/NetworkManager.h#L17-L31
Events: https://github.com/espressif/arduino-esp32/blob/master/libraries/Network/src/NetworkEvents.h#L143-L219

Network.setDefaultInterface(NetworkInterface &ifc); takes as argument STA, ETH or PPP
This allows to set which of those is used for Internet.