feat(netlink): detect ipv6 support level (#2523)

- add option `IPV6_CHECK_ADDRESSESES=[2001:4860:4860::8888]:53,[2606:4700:4700::1111]:53`
- gluetun needs access to the addresses above through the host firewall, to test ipv6 support before setting up the vpn

internal/cli/noopfirewall.go

@@ -0,0 +1,14 @@
+package cli
+
+import (
+	"context"
+	"net/netip"
+)
+
+type noopFirewall struct{}
+
+func (f *noopFirewall) AcceptOutput(_ context.Context, _, _ string, _ netip.Addr,
+	_ uint16, _ bool,
+) (err error) {
+	return nil
+}