[Feat] Routing specific network interface by data network name

[ntut-xuan]

Hi I'm Uriah.

I notice that we have a interface field in UPF DNN. By tracing the code, I realize it doesn't used on anywhere and seems that it just reserved a field for routing specific network interface by data network name. [1]

It would be good to have this feature and I'm welling to contribute it. Please provide me some advice if I want to contribute this feature.

Reference 1: config.go