To help generic code, an interface of an event queue at the PAL level has been
added. Implementation can either rely on the event mechanism internal to the
stack or use the SimpleEventQueue implementation provided by this patch.
This interface expose the primitives needed to realize operations defined in
the GAP layer. Data types, event and function definitions follow closely HCI
commands and events defined in the Bluetooth specification.
This changes introduce a platform adaptation over ATT/GATT that can be implemented by porter.
Unlike the GattClient interface, the ATT/GATT adaptation is simple, follow closely the Bluetooth specification and won't change over time.
Implementation of the GattClient interface is realized by the class GenericGattClient which accept in input a pal::GattClient.
This change will also free design space once adopted by partners, addition to the GattClient interface won't require partner support.