Add formFactor filter for operators graphql query

[hbruch]

This PR adds formFactor as optional filter to the graphql operators query.

An operator matches a given formFactor, if any vehicle_type it rents has of the requested formFactors.

Note: this filter evaluates vehicle_types, not currently available vehicle_types.

This PR fixes #215.

[testower]

The vehicleTypes property of FeedProvider is intended as a way to hard code static vehicle type data from feedproviders.yml if the upstream data source does not provide it. It is not populated with actual data from upstream data sources.

[hbruch]

Oh, I see. Will explore, if GBFSFeedCache can be used, or do you have another suggestion?

[testower]

Yeah, you could of course iterate over all systems and fetch their vehicle_types.json from the cache, and aggregate the data from there, but it is a bit brute.

I do have some ideas on how this should be solved, but I need to flesh it out a bit with some drawing. I think it involves adding the concept of an Operator entity.

Are you available next week for a call and we can discuss it?