Remove recursion from LocalRuntime during execution of FLFlow steps

[KeertiX]

TITLE: Remove recursion from LocalRuntime during execution of FLFlow steps

SUMMARY OF CHANGES:
Currently FLFlow steps are executed in a nested manner. This PR attemppts to remove recursion from LocalRuntime,
following approach is followed:

1. call start()
2. call execute_task
   - Function is executed till flow ends and arguments to execute the next step are updated by the FLSpec.next() method
   - This method is now split into 3 parts based on type of steps being executed:
   • execute_foreach_task: For executing collaborator steps
   • execute_agg_task: For executing aggregator steps
   • execute_end_task: For executing the last step in the flow

VERIFICATION SUMMARY:

• Changes verified in Jupyter notebook and python file, with ray and single process backend

NEXT STEPS:

• Further optimization to avoid serialization of clones in every collaborator step