Possibility to query systems based on operator in the mobility GQL api

[gorandalum]

Is your feature request related to a problem? Please describe.
In the AtB app we have a need to redirect to an operators app without selecting a specific vehicle or station first. This is not possible as of now as the system information is only available as a subfield on vehicle/station.

Goal / high level use-case
We in AtB have deals with some mobility-companies where users get some free usage of their vehicles if they have purchased periodic ticket in our app. This works by adding a bonus-code as a query-param when linking to the operator app. As of now we can't make this feature as visible as we want, as we are only able to link to the operator app after the user has selected a specific vehicle or station in the app.

Describe the solution you'd like
Query system directly, filtered by operator id.

Describe alternatives you've considered
Just querying stations by operator id to find a random station, and use the system from that station as a source for URLs. The problem is that also lat/lon is mandatory. We don't have coordinates to send in when user is not in the map, and sending in some random coordinates (like Trondheim centrum) won't work since it might be no vehicles/stations for that operator in that area.