Bluetooth: GATT: Add option to enforce client change-unware state

This adds BT_GATT_ENFORCE_CHANGE_UNAWARE option which when enable
returns -EAGAIN when notifying or indicating if the client is
change-unware to conform with following statement on the spec:

  'BLUETOOTH CORE SPECIFICATION Version 5.1 | Vol 3, Part G page 2350:
   Except for the Handle Value indication, the  server shall not send
   notifications and indications to such a client until it becomes
   change-aware.'

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>