gatt/gap: Read Device Name Characteristic

This commit adds logic to discover Device Name Characteristic from
GAP Service, read it and set the btd_device name using the response.
gatt/gap: Read Appearance Characteristic

This commit adds logic to discover Appearance Characteristic from
GAP Service, read it and set the btd_device appearance using the
response.
TO SQUASH with "gatt: Introduce helper for registering a notifier"
TO SQUASH with "gatt: Introduce helper for unregistering a notifier"
WIP gatt/gap: Handle Service Changed indication