Creates a new custom field in a workspace. Every custom field is required
to be created in a specific workspace, and this workspace cannot be
changed once set.

A custom field’s name must be unique within a workspace and not conflict
with names of existing task properties such asDue Date orAssignee.
A custom field’s type must be one oftext,enum,multi_enum,number,
date, orpeople.

Returns the full record of the newly created custom field.