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`

.github/labels.yml

@@ -70,6 +70,9 @@
 - name: "Openvpn"
   color: "ffc7ea"
   description: ""
+- name: "Wireguard"
+  color: "ffc7ea"
+  description: ""
 - name: "Unbound (DNS over TLS)"
   color: "ffc7ea"
   description: ""