Added Multihoming feature to LWIP (ability to use more than one network interfaces) for increasing networking reliability.
This involves:
LWIP interface
LWIP IP routing
DNS storage
Sockets (bind to interface name possibility)
possibility to add non default network interface
cellular middleware modifications if cellular connection is used
Allow more than one callback to be register to NetworkInterfaces.
This introduces new APIs:
void NetworkInterface::add_event_listener(...);
void NetworkInterface::remove_event_listener(...);
Which internally calls interfaces attach() functions.
Most functions are empty or simply return "UNSUPPORTED", but it is still worth covering this functions with unit tests to have better control of unwanted changes.