Change TrainSchedule model for arrival on stop signal

[woshilapin]

Description and goal

Depends on OpenRailAssociation/osrd-website#188

The arrival on stop signal must be configurable, and therefore, must appear in the TrainSchedule model. See the design documentation for how it should be done.

Acceptance criteria:

• [ ] the model can take the instruction on a stop
• [ ] the option (arrival on stop signal) should only be enabled on full stops, not route points (otherwise, request is rejected)