Added onnx export support for CopyColumns

[harishsk]

Fixes #4218

[ganik]

[codemzs]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[codecov]

Codecov Report

Merging #4486 into master will increase coverage by 0.01%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master    #4486      +/-   ##
==========================================
+ Coverage    74.9%   74.92%   +0.01%     
==========================================
  Files         908      908              
  Lines      160072   160097      +25     
  Branches    17222    17226       +4     
==========================================
+ Hits       119903   119947      +44     
+ Misses      35359    35344      -15     
+ Partials     4810     4806       -4

Flag	Coverage Δ
#Debug	74.92% <96.55%> (+0.01%)	⬆️
#production	70.26% <80%> (+0.01%)	⬆️
#test	90.28% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
test/Microsoft.ML.Tests/OnnxConversionTest.cs	95.04% <100%> (+0.12%)	⬆️
src/Microsoft.ML.Data/Transforms/ColumnCopying.cs	96.15% <80%> (+10.71%)	⬆️
...ML.Transforms/Text/StopWordsRemovingTransformer.cs	86.26% <0%> (+0.15%)	⬆️
...soft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs	85.31% <0%> (+0.2%)	⬆️
src/Microsoft.ML.AutoML/Sweepers/Parameters.cs	83.05% <0%> (+0.84%)	⬆️
....ML.AutoML/PipelineSuggesters/PipelineSuggester.cs	83.19% <0%> (+3.36%)	⬆️

[harishsk]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).