feat(ev): add charging priorities and reservations

[sebhoerl]

This PR makes three modifications to the ev contrib:

Testing

• Some classes are made public as we need them to construct comprehensive unit tests for the ev contrib in later PRs.

Charging priorities

• The ChargingPriority is introduced: it is used for managing the charging queue, which, currently, is a simple FIFO. With ChargingPriority we now traverse the queuing vehicles and let them pass on to charging if the ChargingPriority returns true. This means that we can retain certain vehicles if others should get plugged with a higher priority than them.

Reservations

• A useful implementation of ChargingPriority is provided, which is ReservationBasedChargingPriority
• As soon as a vehicle enters the queue of a charger, it is checked whether there is space and whether the vehicle has a reservation for the current time. Only if this is the case, the vehicle can directly pass on to charging.
• If no reservation is found, the ChargingPriority tries to make a reservation ad-hoc. If this succeeds, the vehicle is also allowed to plug.
• If no external intervention is happening (removing the vehicle from the charger, making a manual reservation), the vehicle will queue until, eventually, a spontaneous reservation can be made.
• The main purpose, however, is that some external logic (for instance, let's say, dvrp charging planning) can explicitly make reservations in advance using the singleton service ReservationManager. Likewise, any external logic can check in advance if a charger is already fully booked at a future time.