Add assignment controlled_load_balance. Add option to send table modify commands

assignments/controlled_load_balance/Makefile

assignments/controlled_load_balance/README.md

assignments/controlled_load_balance/balance_controller.py

assignments/controlled_load_balance/receive.py

assignments/controlled_load_balance/s2-runtime.json

assignments/controlled_load_balance/s3-runtime.json

assignments/controlled_load_balance/send_many.py

assignments/controlled_load_balance/topology.json

utils/p4runtime_lib/helper.py

utils/p4runtime_lib/switch.py