Return a list of system order types

Merchants can create custom Order Types via "/v3/merchants/{mId}/order_types". It is useful to associate these custom order types with particular system order types in order to group things functionally. For example, a merchant may have a "Lunch Take-Out" order type and a "Dinner Take-Out" order type. These two order types can be associated with the "TAKE-OUT-TYPE" system order type so that applications can understand that they are both take-out order types.