Returns a list of all custom fields for the given organization. Custom fields can be created by the organization via the Pliant web app or API. Those fields can be used to store additional information on transactions, e.g. an additional field needed for pre-accounting purposes, or a reference the organizations needs to reconciliation processes later in their systems. Custom fields can be set on card- or transaction-level. If set on card-level, each transaction made with this card gets a copy of the custom field values of the respective card.