add checkpoint apis

[galalen]

Description

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Affected Dependencies

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How has this been tested?

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Checklist

• [ ] I have followed the Contribution Guidelines and Code of Conduct
• [ ] I have commented my code following the OpenMined Styleguide
• [ ] I have labeled this PR with the relevant Type labels
• [ ] My changes are covered by tests

[codecov[bot]]

Codecov Report

Merging #319 (ae3cfed) into dev (c58854b) will increase coverage by 0.92%. The diff coverage is 34.67%.

@@            Coverage Diff             @@
##              dev     #319      +/-   ##
==========================================
+ Coverage   30.83%   31.76%   +0.92%     
==========================================
  Files          54       56       +2     
  Lines        1456     1571     +115     
  Branches      229      249      +20     
==========================================
+ Hits          449      499      +50     
- Misses        964     1025      +61     
- Partials       43       47       +4     

Impacted Files	Coverage Δ
.../main/java/org/openmined/syft/execution/SyftJob.kt	14.06% <0.00%> (-5.09%)	:arrow_down:
...java/org/openmined/syft/execution/TrainingState.kt	0.00% <0.00%> (ø)
...mined/syft/execution/checkpoint/ModelCheckPoint.kt	68.75% <68.75%> (ø)
.../syft/execution/checkpoint/CheckPointSerializer.kt	72.72% <72.72%> (ø)
...enmined/syft/networking/datamodels/ClientConfig.kt	20.00% <0.00%> (+20.00%)	:arrow_up:

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