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/formatting.go

@@ -19,6 +19,8 @@ var ErrNoServerFound = errors.New("no server found")
 func NoServerFoundError(selection configuration.ServerSelection) (err error) {
 	var messageParts []string
 
+	messageParts = append(messageParts, "VPN "+selection.VPN)
+
 	protocol := constants.UDP
 	if selection.OpenVPN.TCP {
 		protocol = constants.TCP