core(routing): use less brittle route reconciliation mechanism

[encodeous]

Currently, nylon wraps the ip command on linux, /sbin/ifconfig on mac, and netsh on Windows.

This is not problematic in itself, however nylon also keeps a model of the system route table in-memory, without actually retrieving the system routing table. This can lead to drift over time, if there are other applications/users which modify the route table, or if a command fails for some reason.

To resolve: implement a cross-platform route table interface, like:

type SystemRoutes interface {
      InterfaceAddresses(ifName string) ([]netip.Prefix, error)
      AddAddress(ifName string, addr netip.Addr) error
      DeleteAddress(ifName string, addr netip.Addr) error

      InterfaceRoutes(ifName string) ([]netip.Prefix, error)
      AddRoute(ifName string, prefix netip.Prefix) error
      DeleteRoute(ifName string, prefix netip.Prefix) error
}

I looked online, and I don't think there is such an "all-in-one" go library for the major platforms.

[encodeous]

Currently, we have to rely on hacks like: #98