Add the "interface" method to get public IP

[kaoet]

Some ISPs do NAT on the way to public IP discovery services like icanhazip.com.

This MR adds a new method to get public IP, which is to get it from IP of primary interface directly.

[adrienbrignon]

Hey, thanks for your contribution.

So if I understand, it'll work for devices which are not behind a NAT?

[kaoet]

Yes. I have a globally routable IPv4 address on my router. But my ISP is really bad. There is NAT on the way to some websites, including icanhazip.com. I can't get correct IP with them.