The Bluetooth SIG website hosts several descriptions of standard services that Bluetooth Low-Energy devices could implement. It provides this description in a human readable format, but also implies that there is a machine-readable version of these schemas called "GATT XML". I cannot find the specification for GATT XML anywhere -- does it exist? If not, is there another format for BLE service schemas?
It does in fact exist. The XML schema is available for download here: https://www.bluetooth.com/specifications/gatt/schema