Invalid vehicle_type_id not detected

[ThorstenFroehlinghaus]

https://api.mobidata-bw.de/sharing/gbfs/v2/donkey_le_locle/free_bike_status references vehicle_type_ids like DKP:VehicleType:bike which are not contained https://api.mobidata-bw.de/sharing/gbfs/v2/donkey_le_locle/vehicle_types. These inconsistencies are not reported in https://api.mobidata-bw.de/sharing/validation/systems. Such violations of foreign key constraints shall be listed in the validation result.

[hbruch]

Such a check has been added via https://github.com/entur/gbfs-validator-java/pull/114