Bluetooth: Introduce new bt_set_id_addr() API

There are certain use cases where the application needs to be able to
explicitly set a specific identity address. This was previously
possible using the bt_storage API, however now that it's gone another
solution is needed.

This patch adds a ne bt_set_id_addr() API which the application can
use to set a specific identity address before calling bt_enable().

Fixes #7434

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>