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/updater/providers/windscribe/api.go

@@ -29,6 +29,7 @@ type groupData struct {
 	City     string       `json:"city"`
 	Nodes    []serverData `json:"nodes"`
 	OvpnX509 string       `json:"ovpn_x509"`
+	WgPubKey string       `json:"wg_pubkey"`
 }
 
 type serverData struct {