Add created_at timestamp to feed response

[emmambd]

Describe the problem

In order to track what feeds have been added over time, we need to add a created_at timestamp to a feed.

Proposed solution

• Add created_at timestamp to a feed, including serving it in the feeds, gtfs and gtfs-rt feed responses. Description should state that this timestamp is when it was added to the Mobility Database backend.
• Manually updated the created_at timestamps for previous feeds. Spreadsheet of dates after February 8th here.

Alternatives you've considered

No response

Additional context

No response

[cka-y]

Tasks:

• [x] Modify DB schema
• [x] Modify populate script
• [x] Modify API response for schedule and realtime feeds
• [x] Add/modify unit test
• [ ] Create sql script to manually update existing feeds