add Flower for federated learning

[mariaboerner1987]

What is this Python project?

The open sozúrce project Flower allows to federate any workload with any machine learning framework. It can be used to develop federated learning, analytics, and evaluation.

What's the difference between this Python project and similar ones?

Adding federated learning resources.

--

Anyone who agrees with this pull request could submit an Approve review to it.