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/netlink/mocks_test.go

@@ -0,0 +1,50 @@
+// Code generated by MockGen. DO NOT EDIT.
+// Source: github.com/qdm12/gluetun/internal/netlink (interfaces: Firewall)
+
+// Package netlink is a generated GoMock package.
+package netlink
+
+import (
+	context "context"
+	netip "net/netip"
+	reflect "reflect"
+
+	gomock "github.com/golang/mock/gomock"
+)
+
+// MockFirewall is a mock of Firewall interface.
+type MockFirewall struct {
+	ctrl     *gomock.Controller
+	recorder *MockFirewallMockRecorder
+}
+
+// MockFirewallMockRecorder is the mock recorder for MockFirewall.
+type MockFirewallMockRecorder struct {
+	mock *MockFirewall
+}
+
+// NewMockFirewall creates a new mock instance.
+func NewMockFirewall(ctrl *gomock.Controller) *MockFirewall {
+	mock := &MockFirewall{ctrl: ctrl}
+	mock.recorder = &MockFirewallMockRecorder{mock}
+	return mock
+}
+
+// EXPECT returns an object that allows the caller to indicate expected use.
+func (m *MockFirewall) EXPECT() *MockFirewallMockRecorder {
+	return m.recorder
+}
+
+// AcceptOutput mocks base method.
+func (m *MockFirewall) AcceptOutput(arg0 context.Context, arg1, arg2 string, arg3 netip.Addr, arg4 uint16, arg5 bool) error {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "AcceptOutput", arg0, arg1, arg2, arg3, arg4, arg5)
+	ret0, _ := ret[0].(error)
+	return ret0
+}
+
+// AcceptOutput indicates an expected call of AcceptOutput.
+func (mr *MockFirewallMockRecorder) AcceptOutput(arg0, arg1, arg2, arg3, arg4, arg5 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "AcceptOutput", reflect.TypeOf((*MockFirewall)(nil).AcceptOutput), arg0, arg1, arg2, arg3, arg4, arg5)
+}