Bluetooth: Host: Define bt_smp_err_to_str()

This API converts a SMP error code to a string.
This can be useful if application developers want
to print them in the applications.

BT_SMP_ERR_SUCCESS was added for completeness.

Later we can also use them in the host to improve debuggability.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>