After the router starts, we need to run some external command.

We now use PostExec (https://github.com/arduino/arduino-router/blob/main/debian/arduino-router/etc/systemd/system/arduino-router.service#L12) in the systemd service. Although we should only execute this command right after the router is really ready to receive traffic, and not after the process starts.

We should add a new flag that permits passing an arbitrary command that will be executed right after the router is ready for traffic. For instance

arduino-router --exec-after='/usr/bin/gpioset -c /dev/gpiochip1 -t0 70=1'

As a part of this task, we should also use the aforementioned flag instead of the PostExec in the systemd service https://github.com/arduino/arduino-router/blob/main/debian/arduino-router/etc/systemd/system/arduino-router.service