Add API for list of vehicle names

[rajat2004]

Settings -

{
    "SettingsVersion": 1.2,
    "SimMode": "Car",

    "Vehicles": {
        "Car1": {
          "VehicleType": "PhysXCar"
        },
        "Car2": {
          "VehicleType": "PhysXCar",
          "X": 0, "Y": 10, "Z": 0
        }
    }
}

API -

>>> client.listVehicles()
['Car1', 'Car2']
>>> 

Came to mind in #2921, just wrote it since was straightforward to implement

The keys contain an empty string, coming from it being used to refer to the default vehicle, see ApiProvider.hpp being called from SimModeBase.cpp
A simple way is to just remove the empty string ourselves, which is being done right now.
However, the API could also mean - return a list of valid vehicle names which can be passed to APIs, in which case empty string is valid. But it could also introduce more complexity to the client code, such as removing the empty string if data of each vehicle is required only once, and having an empty string will duplicate calls for one vehicle.
I'm leaning towards removing the empty string, and will add that behaviour in the PR. But do comment if the second behaviour makes more sense.

Could be particularly useful if #2390 goes in (which I hope to update in the next few days)
Not much of an use-case otherwise, just allows the user to avoid parsing the settings for getting the vehicle names

[rajat2004]

Rebased on master, conflict in every file (ouch), didn't go well with the Wind PR
But seems like both Travis and Azure Pipelines have stopped, would be good to have that looked at, don't want to go back to when master didn't compile on different platforms (which I myself caused a few times 😅 )
Ping @saihv @madratman

[zimmy87]

Hey @rajat2004, can you resolve the current conflicts to get this PR ready for merging?

[zimmy87]

Tested Unreal & Unity builds, API worked as expected in both builds, so I am moving ahead with merging this.