Bluetooth: GATT: Introduce BT_GATT_ATTRIBUTE

This adds BT_GATT_ATTRIBUTE which replaces BT_GATT_DESCRIPTOR as the
most basic way to declare an attribute since using descriptor may be
confusing when declaring things like a characteristic value since the
Bluetooth has their procedures completely separated.

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