Experimental Workflow interface

[psfoley]

Closes #627

This PR adds:

• A new workflow interface (inspired by the Metaflow project) - Define a chain of tasks to be performed on aggregator or collaborator.
• Runtimes - A runtime defines the infrastructure where the workflow will be executed.
  • LocalRuntime - Run the workflow on a single node
    • Two backends supported: Execute a flow in a single process (default) or in multiple processes (for concurrent execution) with Ray
  • Additional runtimes supporting multi-node execution to be made available in a future release
• Partial compatibility with Metaflow Client for enhanced federated workflow debugging (note: all of these are optional)
  • Save Data Artifacts
  • Save stdout / stderr
  • Generate Workflow DAG that can be loaded in HTML (only currently compatible in flows without loops)
• New Use Cases / Tutorials under the openfl-tutorials/experimental folder:
  • Aggregator Validation after Collaborator Training
  • Model Watermarking
  • Privacy Meter - Measuring information leakage after model training
  • Global Differential Privacy
  • Vertical FL Examples
  • Cyclic Institutional Incremental Training

Remaining items before removing WIP tag:

• Refactor logic in LocalRuntime
• Merge PRs that ensure no leakage between tasks
• Selective CI Pipelines for PRs labelled with workflow_interface
• Comprehensive documentation
• Review and update licenses

This PR includes contributions from:
@mansishr, @anindya-paul, @brandon-edwards, @changhongyan123, @scngupta-dsp, @ParthM-GitHub, @Greesh-N, @KeertiX, @igor-davidyuk, @aleksandr-mokrov, @itrushkin, @msheller, @hanboa

[psfoley]

Partial feedback on code reviewed so far:

• Please use typehints across functions, unless the function's behaviour is implicit or obvious. Typehints are a documentation in disguise.
• Most functions have missing docstrings (by that, I mean an account of accepted values of an argument, default values etc., not just a literal expansion of what the function does)
• Given this new interface and growth of this repository, it would make sense to "consider" moving examples to a separate repository, and tag releases that are supposed to work for examples.

@MasterSkepticista thanks for the feedback! I've added type hints and expanded docstrings for user facing functions. @anindya-paul is working on some of the other comments specific to Global DP and Privacy Meter.

[psfoley]

It is nice the PR comes with a vertical FL example. I am wondering if it would be possible to implement cyclic training and cross-site validation schemes with FL Flow interface.

@igor-davidyuk Excellent idea. I've added a cyclic training example notebook to this PR here that compares that approach with standard horizontal FL under various IID / Non-IID scenarios: https://github.com/intel/openfl/blob/554f674b71b7bc5346e65a98752d4959210492eb/openfl-tutorials/experimental/Workflow_Interface_103_Cyclic_Institutional_Incremental_Learning.ipynb

[mansishr]

LGTM, great work!