Allow vehicles to lead (e.g. with LongitudinalDistanceAction)

Describe the feature

The LongitudinalDistanceAction allows a controller-controlled entity to follow another entity at a fixed distance or time gap along that entities trajectory or route. This is useful for simulating non-ego vehicles following each other or the ego vehicle.

(I am assuming a positive distance means the actor is following another vehicle, it's actually quite hard to tell from the documentation).

Another useful combination would be defining a non-ego who leads ahead of the ego at a fixed time or distance (e.g. to test emergency brake situations).

Finally (could be split as a separate issue), we should be clear what a positive and a negative distance mean here (which means to lead and which means to tail).

Describe the solution you would like

LongitudinalDistanceAction allow both positive and negative distances to be specified. The meaning of each is clearly defined in the object model.

Describe alternatives you have considered

Not sure there is one..

Describe the backwards compatibility

Fully backwards compatible as we have only ever allowed positive distances in each.

Additional context

N/A