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/configuration/settings/ipv6.go

@@ -0,0 +1,58 @@
+package settings
+
+import (
+	"net/netip"
+
+	"github.com/qdm12/gosettings"
+	"github.com/qdm12/gosettings/reader"
+	"github.com/qdm12/gotree"
+)
+
+// IPv6 contains settings regarding IPv6 configuration.
+type IPv6 struct {
+	// CheckAddresses are the TCP ip:port addresses to dial to check if
+	// IPv6 is supported, in case a default IPv6 route is found.
+	// It defaults to google and cloudflare IPv6 anycast addresses
+	// [2001:4860:4860::8888]:53,[2606:4700:4700::1111]:53
+	CheckAddresses []netip.AddrPort
+}
+
+func (i IPv6) validate() (err error) {
+	return nil
+}
+
+func (i *IPv6) copy() (copied IPv6) {
+	return IPv6{
+		CheckAddresses: gosettings.CopySlice(i.CheckAddresses),
+	}
+}
+
+func (i *IPv6) overrideWith(other IPv6) {
+	i.CheckAddresses = gosettings.OverrideWithSlice(i.CheckAddresses, other.CheckAddresses)
+}
+
+func (i *IPv6) setDefaults() {
+	defaultCheckAddresses := []netip.AddrPort{
+		netip.MustParseAddrPort("[2001:4860:4860::8888]:53"),
+		netip.MustParseAddrPort("[2606:4700:4700::1111]:53"),
+	}
+	i.CheckAddresses = gosettings.DefaultSlice(i.CheckAddresses, defaultCheckAddresses)
+}
+
+func (i IPv6) String() string {
+	return i.toLinesNode().String()
+}
+
+func (i IPv6) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("IPv6 settings:")
+	addrsNode := node.Appendf("Check addresses:")
+	for _, addr := range i.CheckAddresses {
+		addrsNode.Append(addr.String())
+	}
+	return node
+}
+
+func (i *IPv6) read(r *reader.Reader) (err error) {
+	i.CheckAddresses, err = r.CSVNetipAddrPorts("IPV6_CHECK_ADDRESSES")
+	return err
+}