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/address.go

@@ -0,0 +1,25 @@
+package wireguard
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/vishvananda/netlink"
+)
+
+func (w *Wireguard) addAddresses(link netlink.Link,
+	addresses []*net.IPNet) (err error) {
+	for _, ipNet := range addresses {
+		address := &netlink.Addr{
+			IPNet: ipNet,
+		}
+
+		err = w.netlink.AddrAdd(link, address)
+		if err != nil {
+			return fmt.Errorf("%w: when adding address %s to link %s",
+				err, address, link.Attrs().Name)
+		}
+	}
+
+	return nil
+}