Allow keeping rental bicycles at the destination

[flaktack]

In certain cases it can be useful to keep a (station) rental bicycle at the destination without returning it, which currently is not possible.

Use cases would be:

• bicycle rental networks with a single station, where the bicycle has to be returned to the original rental station (roundtrips)
• "intermediate" places-like behavior, where the bike will continue to be used, and so doesn't need to be returned at the end of the planned itinerary.

For this to be supported a few things would need to be changed:

• adding a parameter to the GBFS bike rental updater if this functionality should be enabled (allowKeepingRentedBicycleAtDestination)
• allowing the functionality to be enabled in the API (RoutingRequest#allowKeepingRentedBicycleAtDestination), along with a (RoutingRequest#keepingRentedBicycleAtDestinationCost) to add a cost for such trip plans so that returning the bike to a nearby station could be preferred
• adding a flag to itineraries to signify that it ends with rented bicycle in possession (Itinerary#arrivedAtDestinationWithRentedBicycle)