Heterogeneous Fleets

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Support different types of vehicles such as walk, bike, car.
Could also be used to model "skill sets" when dispatching employees.

This requires the user to hand us vehicle-class specific
- cost matrices
- duration matrices
- capacities

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Implementation:

For Costs:

Use `SetArcCostEvaluatorOfVehicle` and provide callback on a per-vehicle basis. We now require numVehicle cost matrices from the user.

For Time Dimension:

Use `AddDimensionWithVehicleTransits`. Takes a vector of transition time evaluators _per vehicle_ for pairs of nodes. `Evaluator(i, j) { return ServiceTime(i) + TransitionTime(i, j); }`. We now require numVehicle duration matrices from the user.

For Capacity Dimension:

Use `AddDimensionWithVehicleCapacity`. See explanation above.

For Time and Capacity:

Use `AddDimensionWithVehicleTransitAndCapacity`. See explanation above.

Start from the vehicle-routing entry points exposing SetArcCostEvaluatorOfVehicle, AddDimensionWithVehicleTransits, AddDimensionWithVehicleCapacity, and AddDimensionWithVehicleTransitAndCapacity. Trace how cost, duration, and capacity inputs are currently represented, then verify that each supports per-vehicle matrices and heterogeneous fleets.

Written by the indexing model from the issue text.