Retrieve a list of cash drawers

Retrieve a list of the device's connected cash drawers. Discovery of a cash drawer only indicates that it is either connected now, or there is an available hardware port allowing one to be connected. For example, local RJ12-type cash drawers may or may not actually be connected at the time this operation is invoked but will be returned regardless. Other types of cash drawers (e.g. USB cash drawers) may only be discovered if they are physically connected at the time of the call.