Add future_calendar notice

[skalexch]

Describe the problem

We need to notify the producer that their feed does not cover today and is only valid for the future.

This is one of the Google Type notices (TYPE_FEED_FUTURE_SERVICE), was initially requested in #138

Describe the new validation rule

If MIN(MIN(calendar.start_date), MIN(calendar_dates.date)) > today:
Trigger "future_calendar" notice

Sample GTFS datasets

Any dataset marked as "future" in the mobility database.

Severity

INFO

Additional context

@emmambd We need to decide whether a feed with future service (calendar.txt or calendar_dates.txt) but current validity in feed_info.txt should trigger the notice or not.

In the legacy validator it's valid since the earliest date comes from whichever file (calendar or calendar_dates or feed_info), so if the service in calendar.txt is a future service but the dates in feed_info.txt cover today then this notice will not be triggered).

We decided to split into two notices (two issues):

• One notice when the calendar+calendar dates that are fully in the future -> This issue.
• One notice when the feed validity is fully in the future -> Add future_feed notice #2067