make BleakGATTService, BleakGATTCharacteristic, BleakGATTClient platform agnostic

[dlech]

Currently, there is a backend-specific implementation for BleakGATTService, BleakGATTCharacteristic and BleakGATTClient. However, these should only contain static data, so all data can be passed into the constructor instead of overriding properties. The obj property is the only platform-dependent property an can be typed as typing.Any.

These types can then be moved out of the backends module (as suggested in #904 (comment)).

The non-public methods like get_descriptor and add_descriptor should be made private (by adding an underscore prefix) when we do this.

Also, new properties should be added (as suggested in #759 (comment)) to navigate back up the tree. For example, the descriptor class should have a characteristic property to get the parent characteristic and the service class should have a client property to get the BleakClient. When this is done, we can deprecate the properties like service_uuid since we will be able to write service.uuid instead.