QLowEnergyService::discoverDetails should become optional

In https://bugreports.qt.io/browse/QTBUG-75340 we changed QLowEnergyService::discoverDetails() to be configurable: A parameter was introduced to control whether all characteristics and descriptors are read.

This item is about making discoverDetails() optional.

At the moment, a QLowEnergyService is not usable before discoverDetails(). When this is done, it is usable.

At the moment, a SkipValueDiscovery makes it usable. When this is done, a SkipValueDiscovery does nothing.

At the moment, a FullDiscovery makes it usable and reads all characteristics and descriptors. When this is done, a FullDiscovery only reads all characteristics and descriptors.