Add CLI command "aignostics system online" which exits with 1 if not online, 0 otherwise, and prints info if online (green) or not (red).

For that provide is_online() in system service, drawing from system info, as in using well known and reliable source to check connectivity.

Cache online information for 60 seconds (default), with that number configurable in settings of system module.

Provide tests for the above in tests directory of system module.