Implement feed dates verification (GTFS rule)

[maximearmstrong]

Is your feature request related to a problem? Please describe.
A feed must have dates linked to it, and must have exceptions for specific dates (trips exceptions, different route, etc.). This is a GTFS rule implemented in Google Python validator and featured in Google Type Error as TYPE_FEED_HAS_NO_SERVICE_DATE_EXCEPTIONS and TYPE_FEED_HAS_NO_SERVICE_DATES.

Describe the solution you'd like
Actual Google GTFS validator behaviour : verifies if feed has effective dates (if empty or is in a valid date format), then verifies service gaps and service exception. Verifies if a feed has exceptions, if the feed has been active for over 6 months.

Describe alternatives you've considered

Additional context
Line 52 et 53 in Error support priorities https://docs.google.com/spreadsheets/d/1vqe6wq7ctqk1EhYkgtZ0_TbcQ91vccfs2daSjn20BLE/edit#gid=0

[isabelle-dr]

We usually don't add rules that are not clearly specified in the spec or best practices (this is the case here), but we make exceptions for what the community thinks is valuable to check (point_near_origin, fast_travel_between_consecutive_stops, unused_trip, just to name a few).
This is done on a case-by-case basis, by discussing with the community.
We are in favor of having the spec modified first before adding the check in the validator.