Allow time gap specification in TrafficSourceAction

Describe the feature

At the moment the only method to define the spawning frequency in the TrafficSourceAction is the "rate" attribute. In OpenPASS there is the requirement to spawn traffic with a distribution of a time gap. So each spawned vehicle has a different time gap to the preceding vehicle. E.g. a uniform distribution between 1 and 3 seconds. So whenever the first vehicle was spawned the next one will be spawned, when the preceding vehicle has reached the time gap for the next vehicle to spawn. This must take the current speed of the preceding vehicle, the initial speed of the next vehicle to be spawned and their distance into consideration.

Describe the solution you would like

Create a new element for time gap distribution and use in an XOR fashion with the existing "rate" attribute. Exact solution still needs to be planned, but the existing distribution classes could be reused.

Describe alternatives you have considered

none.

Describe the backwards compatibility

If we specify an additional element for the time gap distribution, which can be used alternatively to the rate, then the solution is backward compatible.

Additional context

none.