[client] Use native windows sock opts to avoid routing loops (#4314)

- Move `util/grpc` and `util/net` to `client` so `internal` packages can be accessed
 - Add methods to return the next best interface after the NetBird interface.
- Use `IP_UNICAST_IF` sock opt to force the outgoing interface for the NetBird `net.Dialer` and `net.ListenerConfig` to avoid routing loops. The interface is picked by the new route lookup method.
- Some refactoring to avoid import cycles
- Old behavior is available through `NB_USE_LEGACY_ROUTING=true` env var

client/internal/routemanager/util/ip.go

@@ -12,18 +12,8 @@ func GetPrefixFromIP(ip net.IP) (netip.Prefix, error) {
 	if !ok {
 		return netip.Prefix{}, fmt.Errorf("parse IP address: %s", ip)
 	}
+
 	addr = addr.Unmap()
-
-	var prefixLength int
-	switch {
-	case addr.Is4():
-		prefixLength = 32
-	case addr.Is6():
-		prefixLength = 128
-	default:
-		return netip.Prefix{}, fmt.Errorf("invalid IP address: %s", addr)
-	}
-
-	prefix := netip.PrefixFrom(addr, prefixLength)
+	prefix := netip.PrefixFrom(addr, addr.BitLen())
 	return prefix, nil
 }