Create a new user. No payload is required for this request.
When creating a user, you have the option to attach external metadata to that user using the
external_metadata object. This object can be used to attach any external information, such as a customer group identifier for grouping users.
external_metadata object is specified in the POST request, the API saves that information, but only returns the
Metadata can also be updated with a PATCH request.
This request returns the new user's id and an associated user token.