Add CountDistinct and CountIf methods to GroupBy

[sevenzees]

Fixes #7574

Adds a CountDistinct method to the GroupBy class which counts distinct values per group. Adds a CountIf method to the GroupBy class which counts values per group that satisfy a custom predicate.

[codecov]

Codecov Report

❌ Patch coverage is 98.59155% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.03%. Comparing base (25b977e) to head (d936477).

Files with missing lines	Patch %	Lines
src/Microsoft.Data.Analysis/GroupBy.cs	97.05%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7575   +/-   ##
=======================================
  Coverage   69.02%   69.03%           
=======================================
  Files        1482     1482           
  Lines      274099   274169   +70     
  Branches    28266    28272    +6     
=======================================
+ Hits       189199   189263   +64     
- Misses      77518    77523    +5     
- Partials     7382     7383    +1     

Flag	Coverage Δ
Debug	69.03% <98.59%> (+<0.01%)	⬆️
production	63.31% <97.05%> (+<0.01%)	⬆️
test	89.47% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
...st/Microsoft.Data.Analysis.Tests/DataFrameTests.cs	99.90% <100.00%> (+<0.01%)	⬆️
src/Microsoft.Data.Analysis/GroupBy.cs	97.88% <97.05%> (-0.09%)	⬇️

... and 5 files with indirect coverage changes

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