Replace shell-out + string parsing with gopacket/routing

[mortenlj]

We call ip route get 1.1.1.1 and then parse the returned output to extract interface name and IP¹.
This has all sorts of funny edge cases and brittleness, so instead we should see if we can use a library² for this.

Found a blogpost for this exact case:
https://engineering.qubecinema.com/2019/05/13/go-routing-package.html

Footnotes

1. https://github.com/nais/device/blob/master/pkg/wireguard/iptables.go#L11 ↩

2. https://pkg.go.dev/github.com/google/gopacket/routing?utm_source=godoc ↩

[sechmann]

done