Look up vehicle types for operator

[testower]

Is your feature request related to a problem? Please describe.

It would be useful to be able to look up all vehicle types for an operator. This should probably be added to the system entity, and then make available all systems on the operator entity

type Operator {
   ...
   systems: System[]
}

type System {
   ...
   vehicleTypes: VehicleType[]
}

Should probably be considered together with #355

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