Validate conditionally the presence of station_information, station_status and vehicle_status in gbfs.json

[testower]

I believe the following should be possible to express in the schema:

• gbfs.json MUST contain at least one of feed of station_information or vehicle_status. Both files are conditionally required for either dock/dockless systems.
• IF gbfs.json declares a station_information feed, it MUST also declare a station_status feed

[mplsmitch]

Related to this, there have been a few producers that publish feeds for dockless systems which include station_information and station_status files when they have no stations. These files at technically valid but contain garbage data. It would be nice if we could flag these somehow. Here's an example:
https://data.lime.bike/api/partners/v2/gbfs/antwerp/station_information.

Unfortunately I've seen policy documents from multiple cities where they claim that this is a requirement of the specification.