Fairness - balance number of locations serviced per vehicle

Nadie ha tomado este issue todavía.

We want to be able to balance the number of locations serviced by each vehicle to be as even as possible. The use-case is for example drivers who are getting paid (at least partially) by number of deliveries.

This constraint should be behind a flag and optional.

---

Implementation: use the underlying solver to
- Establish a vehicle vars <-> vehicle counts relationship via the `Distribute` constraint
- Add a `Deviation` constraint for vehicle counts targeting the `deviation` var
- Minimizing the `deviation` var

Roughly as follows:

```c++
std::vector vehicleCounts;

for (auto vehicle = 0; vehicle < numVehicles; ++vehicle)
vehicleCounts.push_back(solver->MakeIntVar(0, numNodes));

solver->AddConstraint(solver->MakeDistribute(vehicleVars, vehicleCounts));

auto* deviationVar = solver->MakeIntVar(0, numNodes * numNodes);

solver->AddConstraint(solver->MakeDeviation(vehicleCounts, deviationVar, numNodes));

model.AddVariableMinimizedByFinalizer(deviationVar);
```

No se menciona ningún archivo ni prueba. Empieza localizando el modelo de enrutamiento de vehículos y su configuración de restricciones opcionales, y después inspecciona cómo el solver subyacente expone Distribute, Deviation y la minimización mediante finalizer. Se considera terminado cuando un flag opcional habilita conteos equilibrados de ubicaciones por vehículo y el solver minimiza su desviación.

Escrito por el modelo de indexación a partir del texto del issue.