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/constants/vpn.go

@@ -1,7 +1,8 @@
 package constants
 
 const (
-	OpenVPN = "openvpn"
+	OpenVPN   = "openvpn"
+	Wireguard = "wireguard"
 )
 
 const (