NethVoice: Support external calendars in time conditions

[mgarra]

Description

Add supported calendar management to FreePBX 17, allowing time conditions to use external calendars with reliable automatic synchronization.

Steps to reproduce

1. Configure a Google Calendar in the FreePBX Calendar module.
2. Set the Auto Refresh parameter.
3. Associate the calendar with a time condition.
4. Update an event in Google Calendar and wait for the configured refresh interval.
5. Check the calendar data in FreePBX.

Expected behavior

The external calendar is refreshed automatically by a persistent NS8-compatible service or systemd timer, and the time condition reflects the updated events.

Actual behavior

The Auto Refresh setting does not update the calendar. Synchronization works only when manually running:

fwconsole calendar --sync

A cron job created inside the FreePBX container is not a persistent workaround and may be lost after a container restart or update.

See also

• [Mattermost discussion](https://mattermost.nethesis.it/nethesis/pl/41oxg4urxp863mqy9kg3856dnc)