Improve speed distribution for spawned traffic

Describe the feature

Currently the initial speed definition for spawned traffic is inconsistent between the different traffic actions:

• TrafficSwarmAction: speed range for whole traffic
• TrafficSourceAction: single speed for whole traffic
• TrafficAreaAction: no speed definition

We need to be able to define the initial speed:

• consistently across all traffic actions
• individually for each traffic group
• with statistical distributions

Describe the solution you would like

One solution would be to define the initial speed distribution for each TrafficDistributionEntry or even each EntityDistributionEntry with an additional element that can hold a StochasticDistributionType or a DeterministicSingleParameterDistribution

Describe alternatives you have considered

Define speed via unstandardized properties of TrafficDistributionEntry. Disadvantage: not standardized and initial speed is needed for everyone, so it makes sense to standardize.

Describe the backwards compatibility

depends on implementation. New elements need to be added definetly, so a backward compatible solution via deprecation would be possible.

Additional context

none