feat: move partition consolidator and add LocalAggregator API

[imatiach-msft]

• move PartitionConsolidator API to stages directory
• add LocalAggregator API (to be used by LightGBM)

[imatiach-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[imatiach-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

Merging #1071 (96a2de8) into master (2a716c1) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1071      +/-   ##
==========================================
- Coverage   84.99%   84.90%   -0.10%     
==========================================
  Files         206      206              
  Lines        9751     9751              
  Branches      549      550       +1     
==========================================
- Hits         8288     8279       -9     
- Misses       1463     1472       +9     

Impacted Files	Coverage Δ
...rosoft/ml/spark/stages/PartitionConsolidator.scala	95.74% <ø> (ø)
...ala/org/apache/spark/ml/param/DataFrameParam.scala	69.56% <0.00%> (-17.40%)	⬇️
...a/com/microsoft/ml/spark/io/http/HTTPClients.scala	73.33% <0.00%> (-10.00%)	⬇️

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[imatiach-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).