added FLOptimizer class

[ratmcu]

FLOptimizer that can maintain a optimizer per worker internally.

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[ratmcu]

@iamtrask I added the class and Changed the Notebook to use it.
3.7 Tutorials are failing here on the test
test/notebooks/test_notebooks.py::test_notebooks_basic[examples/tutorials/Part 12 bis - Encrypted Training on MNIST.ipynb] FAILED [ 52%]

[karlhigley]

Duplicate of #3179. Might be worth reviewing #3179 and checking if there aspects from this PR that could be added to that code.

[ratmcu]

@karlhigley , I created this class for arbitrarily creating or calling an optimizer relevant to a model trained in a federated setting. I would like to add any good features from it to the code base with further developments if needed.