Add script handling on modem detection

[Florian Eckert]

Hello,

I am using the modemmanager on openwrt (master).
While playing around, I noticed that it might be useful to trigger
an event after the modemmanager has detected a modem.
We already use '/usr/lib/ModemManager/connection.d' one a successful 
connection.

Background:
I use an APU3 with an EC25, which supports one SIM slot.
However, the APU3 has two sim slots that are switched via a GPIO pin.
After switching, I have to restart the modem so that it recognizes the 
new SIM.
Now it would be good if I could change the SIM credentials after the 
modemmanager
has been detected the rebooted modem and tell the netifd to reestablish
the connection with an 'ifup wwan'.

So far I only see the event in the syslog.

Wed Jul  5 15:06:41 2023 daemon.info [5863]: <info>  [device Modem1] 
creating modem with plugin 'quectel' and '6' ports
Wed Jul  5 15:06:41 2023 daemon.info [5863]: <info>  [base-manager] 
modem for device 'Modem1' successfully created