core: gatt: Export GATT services on D-Bus

This patch adds code that exports an object on D-Bus with the
org.bluez.GattService1 interface for each GATT service that is present
in bt_gatt_client.
core: gatt: Export GATT characteristics on D-Bus

This patch adds code that exports an object on D-Bus with the
org.bluez.GattCharacteristic1 interface for each GATT characteristic
that is present in bt_gatt_client.