Fairness - balance number of locations serviced per vehicle

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 file or test is named. Start by locating the vehicle-routing model and its optional-constraint configuration, then inspect how the underlying solver exposes Distribute, Deviation, and finalizer minimization. Done means an optional flag enables balanced per-vehicle location counts and the solver minimizes their deviation.

Written by the indexing model from the issue text.