feat(server/state): add train getters for already synced fields

[Github-Samuel]

Goal of this PR

Read back a few more train fields server-side. The server already parses them out of the train sync node, there just aren't natives for them yet.

How is this PR achieving the goal

CTrainGameStateDataNode fills in more of CTrainGameStateDataNodeData than the current getters expose. These six are all serialized unconditionally:

s.SerializeSigned(32, 1000.0f, data.distanceFromEngine);

s.Serialize(8, data.trainConfigIndex);
...
s.Serialize(data.isMissionTrain);
s.Serialize(data.direction);
s.Serialize(data.hasPassengerCarriages);
s.Serialize(data.renderDerailed);

s.Serialize(data.forceDoorsOpen);

so this adds:

native	field
GET_TRAIN_CONFIG_INDEX	trainConfigIndex
GET_TRAIN_DISTANCE_FROM_ENGINE	distanceFromEngine
IS_MISSION_TRAIN	isMissionTrain
DOES_TRAIN_HAVE_PASSENGER_CARRIAGES	hasPassengerCarriages
IS_TRAIN_RENDERED_DERAILED	renderDerailed
DOES_TRAIN_FORCE_DOORS_OPEN	forceDoorsOpen

Each one is the same shape as the getters already there, e.g. GET_TRAIN_STATE:

fx::ScriptEngine::RegisterNativeHandler("IS_MISSION_TRAIN", makeEntityFunction([](fx::ScriptContext& context, const fx::sync::SyncEntityPtr& entity)
{
	auto train = entity->syncTree->GetTrainState();

	return train ? train->isMissionTrain : false;
}));

I left the build gated fields out on purpose. carriageSpeed and isTrackDirectionForwards are only serialized under IsWinterUpdate25(), and allowRemovalByPopulation / highPrecisionBlending only under Is2372(), so a getter for those would read an uninitialised member on older builds unless it carried the same gate. stopAtStations is in the same boat and is already handled by DOES_TRAIN_STOP_AT_STATIONS.

DOES_TRAIN_FORCE_DOORS_OPEN reads the per train synced flag, which isn't quite the same thing as the client side SET_TRAINS_FORCE_DOORS_OPEN that sets it globally, so I noted that in its page.

This PR applies to the following area(s)

FXServer, OneSync, Natives

Successfully tested on

Game builds: n/a for the gated fields since none are used; the six added are build independent

Platforms: Windows, Linux

Checklist

• Code compiles and has been tested successfully.
• Code explains itself well and/or is documented.
• My commit message explains what the changes do and what they are for.
• No extra compilation warnings are added by these changes.

I could not do a full server build locally so I left the first box unchecked. What I did check is that each field is actually written by the parser and not behind a build gate, and that none of the six names collide with an existing native in ext/native-decls or the natives stash.