XGBoost Trainer interface (WIP)

[rgesteve]

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Replaces PR #6345 in using a Trainer rather than a Transformer interface for the XGBoost integration. Currently, only a regression trainer is enabled, but a binary classifier is in progress. To run, the user is expected to have a copy of the xgboost library appropriate to their system and available (maybe through a mechanism like updates to their PATH or LD_LIBRARY_PATH environment variables). We are working in ways to reduce the burden of this requirement.

Please expect some edits to this PR, including the addition of unit tests.

[codecov]

Codecov Report

Merging #6383 (5fc1440) into main (ff21cc5) will decrease coverage by 0.00%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6383      +/-   ##
==========================================
- Coverage   68.57%   68.57%   -0.01%     
==========================================
  Files        1171     1171              
  Lines      247197   247197              
  Branches    25799    25799              
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- Hits       169519   169515       -4     
- Misses      70954    70958       +4     
  Partials     6724     6724              

Flag	Coverage Δ
Debug	68.57% <ø> (-0.01%)	⬇️
production	62.96% <ø> (-0.01%)	⬇️
test	88.98% <ø> (ø)

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

Impacted Files	Coverage Δ
...c/Microsoft.ML.FastTree/Utils/ThreadTaskManager.cs	79.48% <0.00%> (-20.52%)	⬇️
src/Microsoft.ML.Maml/MAML.cs	25.88% <0.00%> (-1.02%)	⬇️
src/Microsoft.ML.LightGbm/LightGbmTrainerBase.cs	79.50% <0.00%> (-0.28%)	⬇️
...soft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs	90.25% <0.00%> (+0.47%)	⬆️
src/Microsoft.ML.FastTree/Training/StepSearch.cs	62.37% <0.00%> (+4.95%)	⬆️

[rgesteve]

Replaced in favor of #6662 , which updates to the latest XGBoost version available in miniconda and makes use of .net 7+ features.