Flower Swift SDK

About
The branch implements the Flower SDK for Swift based on the initial draft examples/ios.

Additional
It includes an iOS sample application called FLiOS with benchmarking functionality for testing the SDK.

Major Changes

1. Moving initial implementation of the Swift SDK from examples/ios to src/swift
2. Extension of the existing SDK so that it fits to the proto of Flower 1.0
3. Fixing race conditions and threading bugs
4. Provide a standard implementation of the client protocol within the SDK such that loading data or wrapping layers does not need to be implemented in the client application - unless there is specific need for customization
5. Unit tests of the Swift SDK. It may require extension due to medium test coverage
6. Basic documentation of the Swift SDK code
7. Providing FLiOS - a sample app with benchmark functionalities for testing and benchmarking purposes
8. The file examples/ios/server.py has been streamlined for the sole purpose of running the FL process
9. Included Github Actions to unit test the Swift SDK

How to test
Move to the folder examples/ios and open the file FLiOS.xcodeproj with XCode. Run the application FLiOS on a simulator or a physical device with iOS version 14 or higher. Execute the server/python script examples/ios/server.py. Go through the sequential steps of FLiOS and adjust the server hostname and port accordingly before starting the federated learning process. For test purposes, the standard MNIST sample is used.

Remark
Github Actions for automatically running lint operations to look for possible code flaws, are not included in this PR.