Update register call to use node client

[trex]

On the server-side, we are making the call to FusionAuth to register a user using the fetch API. We can make this request with the FusionAuth node client instead. This will be beneficial if there are updates to the endpoint, as they can get updated with the client, and for consistency.

The reason why some of the other calls to FusionAuth are made using fetch are because the OAuth workflows (login, logout) are not available in the node client.