Specify spawning positions for TrafficAreaAction/TrafficSwarmAction more precisely

Describe the feature

For the TrafficAreaAction and TrafficSwarmAction there is currently only the specification of an area where the entities are spawned. It is not clear where inside this area they should be spawned. This is less a problem to have a standardized behaviour between simulators but to have a reasonable spawning behavior.

Describe the solution you would like

For example we can define that initially for TrafficSwarmAction/TrafficAreaAction entities are spawned in the whole area, but when entities leave the area and get despawned, then new entities are only spawned at the border of the area to avoid entities to pop up out of nowhere for a vehicle under test. Also when spawning around a central entity it doesn't make senase to spawn slower vehicles at the border of the area behind the central entity because they will leave the area right after they were spawned.

Describe alternatives you have considered

Keep spawning positions unspecified and leave it up to simulation developers to implement a reasonable spawning behavior.

Describe the backwards compatibility

Since currently the spawning positions are not defined in more detail, the solution would be not backward compatible for existing spawning implementations in simulators. Probably there are no schema changes necessary, so rom schema perspective the changes are backward compatible.

Additional context

The same question was already brought up as part of #196 here and here