feat(netlink): detect IPv6 using query to address

- If a default IPv6 route is found, query the ip:port defined by `IPV6_CHECK_ADDRESS` to check for internet access

internal/configuration/settings/ipv6.go

@@ -0,0 +1,51 @@
+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 {
+	// CheckAddress is the TCP ip:port address to dial to check
+	// IPv6 is supported, in case a default IPv6 route is found.
+	// It defaults to cloudflare.com address [2606:4700::6810:84e5]:443
+	CheckAddress netip.AddrPort
+}
+
+func (i IPv6) validate() (err error) {
+	return nil
+}
+
+func (i *IPv6) copy() (copied IPv6) {
+	return IPv6{
+		CheckAddress: i.CheckAddress,
+	}
+}
+
+func (i *IPv6) overrideWith(other IPv6) {
+	i.CheckAddress = gosettings.OverrideWithValidator(i.CheckAddress, other.CheckAddress)
+}
+
+func (i *IPv6) setDefaults() {
+	defaultCheckAddress := netip.MustParseAddrPort("[2606:4700::6810:84e5]:443")
+	i.CheckAddress = gosettings.DefaultComparable(i.CheckAddress, defaultCheckAddress)
+}
+
+func (i IPv6) String() string {
+	return i.toLinesNode().String()
+}
+
+func (i IPv6) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("IPv6 settings:")
+	node.Appendf("Check address: %s", i.CheckAddress)
+	return node
+}
+
+func (i *IPv6) read(r *reader.Reader) (err error) {
+	i.CheckAddress, err = r.NetipAddrPort("IPV6_CHECK_ADDRESS")
+	return err
+}