Introduce flower-simulation command and update simulation examples

[jafermarq]

Run simulation with FlowerNext in a single command. SuperLink is terminated once the ServerApp finishes.

• Updates simulation examples so they run in both start_simulation and flower-simulation mode:

  • PyTorch simulation example
  • Tensorflow simulation example <-- #TODO: enable GPU growth for ServerApp
  • Make TF work with flower-simulation launch

Test by:

cd examples/simulation-pytorch
flower-simulation --client-app=sim:client --server-app=sim:server --num-supernodes=100

or:

cd examples/simulation-tensorflow
# append to below `--enable-tf-gpu-growth` if you have a GPU
flower-simulation --client-app=sim:client --server-app=sim:server --num-supernodes=100

Merge first: #3031