Add support for routing objectives

[jmccollum-woolpert]

Let the user control the routing objectives for a scenario.

[johnnychang321-ai]

Start from these ones available in the shipmentmodel.objectives:
https://developers.google.com/maps/tt/route-optimization/experimental/objectives/reference/rpc/google.maps.routeoptimization.v1#type

Allow multiple selections (e.g. checkboxes) per request