feat: let the router set the ready GPIO pin after startup

[cmaglie]

Motivation

Otherwise, the pin may be set high by systemd before the router is actually ready to process data.

Change description

• add a --after-start flag, that executes the given command when the router is ready
• changes the systemd script to let the router set the GPIO pin

Additional Notes

Reviewer checklist

• PR addresses a single concern.
• PR title and description are properly filled.
• Changes will be merged in main.
• Changes are covered by tests.
• Logging is meaningful in case of troubleshooting.