Update JSON Schemas for reconciled state machine

[thekaveman]

After #506 is merged, we'll have a reconciled state machine between Agency and Provider. A follow-up task is to update the JSON Schemas with the new set of vehicle_state and event_type.

If possible, we'd also like to indicate which combinations of those values are considered valid under the state machine as is done now within the status_changes.json Schema. This could be difficult because the new model allows an array of event_type per vehicle_state, and intermediate states are "hidden" by the final valid event transition. See discussion on #271 for more details.

[thekaveman]

As expected, indicating the valid vehicle_state and vehicle_events combinations proved to be a challenge.

In conversation with @karcass, we originally thought that the last entry in the vehicle_events array should be a valid event transition for the indicated vehicle_state, e.g.

{
    "vehicle_state": "non_operational",
    "event_types": ["trip_end", "battery_low"]
}

Here, the battery_low event is a valid transition into the non_operational state (while trip_end is not), and so it comes last in the array.

However, JSON Schema does not support the notion of checking the "last item in an array" - support is for checking a fixed number of items, or all items.

The solution I landed on was to simply check that the event_types array contains a valid event for the given vehicle_state. Thus, both the above example and the following would validate against the JSON Schema:

{
    "vehicle_state": "non_operational",
    "event_types": ["battery_low", "trip_end"]
}

I think this is an OK trade-off in that we'll ensure that each vehicle_state has a correspondingly valid event_type, but we do lose the inherent "ordering" of the event_types array, from the perspective of the JSON Schemas anyway.

[schnuerle]

Thanks @thekaveman that looks like the best solution.

[thekaveman]

I just realized another approach: reverse the order, and have the first array item be the event_type that transitioned to the vehicle_state... so the events are in descending time order instead of ascending. We can revisit if needed.