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/provider/utils/wireguard.go

@@ -0,0 +1,34 @@
+package utils
+
+import (
+	"net"
+
+	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/wireguard"
+)
+
+func BuildWireguardSettings(connection models.Connection,
+	userSettings configuration.Wireguard) (settings wireguard.Settings) {
+	settings.PrivateKey = userSettings.PrivateKey
+	settings.PublicKey = connection.PubKey
+	settings.PreSharedKey = userSettings.PreSharedKey
+	settings.InterfaceName = userSettings.Interface
+
+	const routePriority = 101 // 100 is to receive external connections
+	settings.RulePriority = routePriority
+
+	settings.Endpoint = new(net.UDPAddr)
+	settings.Endpoint.IP = make(net.IP, len(connection.IP))
+	copy(settings.Endpoint.IP, connection.IP)
+	settings.Endpoint.Port = int(connection.Port)
+
+	address := new(net.IPNet)
+	address.IP = make(net.IP, len(userSettings.Address.IP))
+	copy(address.IP, userSettings.Address.IP)
+	address.Mask = make(net.IPMask, len(userSettings.Address.Mask))
+	copy(address.Mask, userSettings.Address.Mask)
+	settings.Addresses = append(settings.Addresses, address)
+
+	return settings
+}