Add limits to objective weights

While [objectives](https://developers.google.com/maps/tt/route-optimization/experimental/objectives/reference/rpc/google.maps.routeoptimization.v1#google.maps.routeoptimization.v1.ShipmentModel.Objective) technically can have any non-negative weight applied to them, it may make sense to cap what the application allows to some reasonable number to provide a better user experience.