FL client worker API

[vvmnnnkv]

Description

Python version of federated learning client.
Fixes #2996

Type of change

Please mark the options that are relevant.

• Added/Modified tutorials
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have added tests for my changes
• I did follow the contribution guidelines
• I have commented my code following Google style.

[review-notebook-app]

Check out this pull request on 

You'll be able to see Jupyter notebook diff and discuss changes. Powered by ReviewNB.

[codecov]

Codecov Report

Merging #3426 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3426      +/-   ##
==========================================
+ Coverage   94.70%   94.72%   +0.01%     
==========================================
  Files         152      152              
  Lines       16038    16038              
==========================================
+ Hits        15189    15192       +3     
+ Misses        849      846       -3     

Impacted Files	Coverage Δ
syft/execution/plan.py	93.90% <0.00%> (+1.21%)	⬆️