Setting a document's custom grand_total

Hello, we'd like to be able to set a `grand_total` on a document; currently the PD frontend is doing this via a `PATCH` call to this endpoint:

```
https://api.pandadoc.com/org/v95ddf9JWAbnmqNMgtpxnm/ws/2mtdqYHPoYr2rYBxpsDTnD/documents/sXjFx6TQnScLEKwA8XFxNk/grand_total
```

But we'd like to have a public/blessed API way of doing this. Thanks!
