post https://app.asana.com/api/1.0/custom_fields
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 as Due Date or Assignee.
A custom field’s type must be one of text, enum, multi_enum, number,
date, or people.
Returns the full record of the newly created custom field.