T3546: PPPoE-server add extended scripts for RADIUS attributes

Ability to get and parse RADIUS attributes via a shell script and
then execute commands
Ane of the usecases is to create a custom shaper with some smart

Extended scripts receive from PPPoE daemon the following variables:

$1 - Interface name
$4 - Tunnel GW IP address
$5 - Delegated IP address to the client
$6 - Calling Station ID (MAC)

if [ -f /run/accel-pppd/radattr.$1 ]; then

true

fi