Add new type to key-value converter

[zewditu]

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So we can do the best job, please check:

• There's a descriptive title that will make sense to other developers some time from now.
• There's associated issues. All PR's should have issue(s) associated - unless a trivial self-evident change such as fixing a typo. You can use the format Fixes #nnnn in your description to cause GitHub to automatically close the issue(s) when your PR is merged.
• Your change description explains what the change does, why you chose your approach, and anything else that reviewers should know.
• You have included any necessary tests in the same PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5e28578) 68.80% compared to head (4571677) 68.80%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6973      +/-   ##
==========================================
- Coverage   68.80%   68.80%   -0.01%     
==========================================
  Files        1249     1249              
  Lines      249686   249686              
  Branches    25485    25485              
==========================================
- Hits       171806   171795      -11     
- Misses      71286    71295       +9     
- Partials     6594     6596       +2     

Flag	Coverage Δ
Debug	68.80% <100.00%> (-0.01%)	⬇️
production	63.27% <ø> (-0.01%)	⬇️
test	88.41% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
test/Microsoft.ML.TestFramework/BaseTestClass.cs	88.23% <100.00%> (ø)
...osoft.ML.Tests/TrainerEstimators/TreeEstimators.cs	97.84% <ø> (ø)

... and 3 files with indirect coverage changes