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/settings_test.go

@@ -67,6 +67,10 @@ func Test_Settings_String(t *testing.T) {
 |       └── Log level: INFO
 ├── Log settings:
 |   └── Log level: INFO
+├── IPv6 settings:
+|   └── Check addresses:
+|       ├── [2001:4860:4860::8888]:53
+|       └── [2606:4700:4700::1111]:53
 ├── Health settings:
 |   ├── Server listening address: 127.0.0.1:9999
 |   ├── Target addresses: