Use `ip monitor` for gateway connectivity

[sonnyp]

Motivation

In #938 we improved connectivity check by replacing ping with arp lookup.

But we still poll every 5 seconds which delays the update of the ip address on the display (and reconfiguring cockpit / mediamtx)

ToDo

We can probably use ip monitor instead. e.g:

async function watchNeighbor(ip, dev = wired_interface, onChange) {
  for await (const line of $`ip --json monitor neigh dev ${dev}`) {
   // bug not actually json 
  // https://bugs.launchpad.net/ubuntu/+source/iproute2/+bug/2116779
    if (line.includes(ip)) {
      const reachable = line.includes('lladdr') && !line.includes('FAILED')
      onChange(reachable)
    }
  }
}