Add federated learning function to the client side

[Zavier-opt]

This is the client side of the federated learning function. Find the full FL function information at the https://github.com/Zavier-opt/fineract-federatedLearning-research/blob/main/READEME.md

The client is responsible for training the model locally under the control of the server. And client side is deployed on the Django rest framework. It is created as a new app(fl_learning) to the previous credit scorecard Django project. Here is the link to the client code: https://github.com/Zavier-opt/fineract-credit-scorecard/tree/develop/fl_learning

This fl_learning app is mainly composed of these files:

views.py: Provides the predict and train function. This is the core of this federated learning function.
fl_model.py: Defines the prediction model, training process, and flower model.
fl_utility.py: Provides utility functions.
urls.py: Define the API.