Add Cars Allowed feature

[skalexch]

Describe the problem

Since we adopted cars_allowed, we might need to include it as a feature.

Base Add-Ons --> Cars Allowed

Proposed solution

If the field cars_allowed exists in at least one entry in trips.txt, then the feature exists.

Alternatives you've considered

No response

Additional context

@emmambd @Sergiodero we need to decide on this feature. We have 66 feeds with "ferry" in the name, and probably a few more others where ferries are part of the agency (needs a script to figure out for now). I estimate around 80-90 feeds that have ferries in total. It might be neat to show if they have information for whether cars are allowed.
@Sergiodero we'd also need to add the feature in GTFS.org