refactor(framework) Remove num-supernodes requirement from simulation exec plugin

[jafermarq]

• It is no longer needed to specify the number of supernodes when launching the SuperLink with the simulation plugin. This means the exec w/ simulation plugin can be now launched as:

flower-superlink --insecure  --executor flwr.superexec.simulation:executor

• A check for num-supernodes in the federation options is performed before creating the Run.