Wireguard support for Mullvad and Windscribe (#565)

- `internal/wireguard` client package with unit tests
- Implementation works with kernel space or user space if unavailable
- `WIREGUARD_PRIVATE_KEY`
- `WIREGUARD_ADDRESS`
- `WIREGUARD_PRESHARED_KEY`
- `WIREGUARD_PORT`
- `internal/netlink` package used by `internal/wireguard`

internal/wireguard/constructor_test.go

@@ -0,0 +1,80 @@
+package wireguard
+
+import (
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func Test_New(t *testing.T) {
+	t.Parallel()
+
+	const validKeyString = "oMNSf/zJ0pt1ciy+qIRk8Rlyfs9accwuRLnKd85Yl1Q="
+	logger := NewMockLogger(nil)
+	netLinker := NewMockNetLinker(nil)
+
+	testCases := map[string]struct {
+		settings  Settings
+		wireguard *Wireguard
+		err       error
+	}{
+		"bad settings": {
+			settings: Settings{
+				PrivateKey: "",
+			},
+			err: ErrPrivateKeyMissing,
+		},
+		"minimal valid settings": {
+			settings: Settings{
+				PrivateKey: validKeyString,
+				PublicKey:  validKeyString,
+				Endpoint: &net.UDPAddr{
+					IP: net.IPv4(1, 2, 3, 4),
+				},
+				Addresses: []*net.IPNet{{
+					IP:   net.IPv4(5, 6, 7, 8),
+					Mask: net.IPv4Mask(255, 255, 255, 255)},
+				},
+				FirewallMark: 100,
+			},
+			wireguard: &Wireguard{
+				logger:  logger,
+				netlink: netLinker,
+				settings: Settings{
+					InterfaceName: "wg0",
+					PrivateKey:    validKeyString,
+					PublicKey:     validKeyString,
+					Endpoint: &net.UDPAddr{
+						IP:   net.IPv4(1, 2, 3, 4),
+						Port: 51820,
+					},
+					Addresses: []*net.IPNet{{
+						IP:   net.IPv4(5, 6, 7, 8),
+						Mask: net.IPv4Mask(255, 255, 255, 255)},
+					},
+					FirewallMark: 100,
+				},
+			},
+		},
+	}
+
+	for name, testCase := range testCases {
+		testCase := testCase
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			wireguard, err := New(testCase.settings, netLinker, logger)
+
+			if testCase.err != nil {
+				require.Error(t, err)
+				assert.Equal(t, testCase.err.Error(), err.Error())
+			} else {
+				assert.NoError(t, err)
+			}
+
+			assert.Equal(t, testCase.wireguard, wireguard)
+		})
+	}
+}