Separate nuget packaging into Stable and Experimental

[maryamariyan]

• I added StableProjects property in a new StablePackagesInfo.props file, which has name of stable projects separated by semicolon.
• I set the strategy for having two separate versions by adding another file next to default BranchInfo.props, called BranchInfo.Stable.props. The files contain version information. Either of the two would be used depending on whether or not a project is listed in StableProjects property.
• I tested the assembly versions and nuget package versions using the diff in this PR.
• The conditions could perhaps be simplified more - needs review

Currently BranchInfo.props generates 0.12.0-preview for all projects. The Major/Minor/Patch versions to set for stable projects are specified in BranchInfo.Stable.props and have identical values to BranchInfo.props until we decide to change them in the near future.

Fixes: #2279
cc: @eerhardt @TomFinley @Ivanidzo4ka @sfilipi @Anipik

[TomFinley]

Thank you @maryamariyan .

[Ivanidzo4ka]

[eerhardt]

We will need to add the other assemblies to the list. And also fix the property so it is statically defined.

[codecov]

Codecov Report

Merging #2951 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2951      +/-   ##
==========================================
+ Coverage   72.27%   72.27%   +<.01%     
==========================================
  Files         796      802       +6     
  Lines      142304   142901     +597     
  Branches    16049    16134      +85     
==========================================
+ Hits       102849   103287     +438     
- Misses      35075    35199     +124     
- Partials     4380     4415      +35

Flag	Coverage Δ
#Debug	72.27% <ø> (ø)	⬆️
#production	68.01% <ø> (-0.01%)	⬇️
#test	88.48% <ø> (+0.06%)	⬆️

Impacted Files	Coverage Δ
...icrosoft.ML.Mkl.Components/MklComponentsCatalog.cs	65.71% <0%> (-28.58%)	⬇️
src/Microsoft.ML.Core/Data/ProgressReporter.cs	70.95% <0%> (-6.99%)	⬇️
...Microsoft.ML.Data/Training/TrainerEstimatorBase.cs	80% <0%> (-4%)	⬇️
src/Microsoft.ML.Core/Data/IEstimator.cs	84.53% <0%> (-2.07%)	⬇️
...oft.ML.StandardTrainers/StandardTrainersCatalog.cs	91.86% <0%> (-1.17%)	⬇️
src/Microsoft.ML.TensorFlow/TensorflowTransform.cs	79.12% <0%> (-0.65%)	⬇️
src/Microsoft.ML.PCA/PcaTrainer.cs	79.94% <0%> (-0.59%)	⬇️
...soft.ML.TestFramework/DataPipe/TestDataPipeBase.cs	73.64% <0%> (-0.36%)	⬇️
src/Microsoft.ML.FastTree/FastTreeRanking.cs	48.19% <0%> (-0.17%)	⬇️
...soft.ML.Transforms/Text/WordEmbeddingsExtractor.cs	87.52% <0%> (-0.03%)	⬇️
... and 389 more

[eerhardt]

Looks great. Thanks for the good work on this, @maryamariyan.