bluetooth: host: Introduce a config option for Service Changed

The Service Changed characteristic support should also work when the
GATT database has been modified after reboot (firmware update scenario)

This commit introduces a BT_GATT_SERVICE_CHANGED config option that is
independent from BT_GATT_CACHING and BT_GATT_DYNAMIC_DB.

Signed-off-by: François Delawarde <fnde@oticon.com>