Metadata: extract feed_start_date and feed_end_date from feed_info

[AntoineAugusti]

Documentation: https://developers.google.com/transit/gtfs/reference#feed_infotxt

These fields would be useful for some GTFS files where service is only running for some months, for example during winter time. Our current tooling treats those GTFS as outdated when the end_date is in the past but we could "know" that it's still valid until a specific date if those fields are present and are in the future.

Example for a network operating only during winter (say in the Alps)

• start_date: 2021-12-01
• end_date: 2021-04-15
• feed_start_date: 2021-11-01
• feed_end_date: 2022-11-01

and the publisher would need to publish a new version of the GTFS before 2022-11-01.