Extend dataframe orderby method to allow defining preferred position for null values

[asmirnov82]

Fixes #7102

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.28571% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 68.49%. Comparing base (07eb681) to head (7585c2a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7118   +/-   ##
=======================================
  Coverage   68.48%   68.49%           
=======================================
  Files        1262     1262           
  Lines      255113   255145   +32     
  Branches    26364    26368    +4     
=======================================
+ Hits       174722   174750   +28     
  Misses      73682    73682           
- Partials     6709     6713    +4     

Flag	Coverage Δ
Debug	68.49% <94.28%> (+<0.01%)	:arrow_up:
production	62.87% <91.48%> (-0.01%)	:arrow_down:
test	88.60% <100.00%> (+<0.01%)	:arrow_up:

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

Files	Coverage Δ
src/Microsoft.Data.Analysis/DataFrame.cs	85.27% <100.00%> (-0.15%)	:arrow_down:
src/Microsoft.Data.Analysis/DataFrameColumn.cs	65.67% <100.00%> (+2.50%)	:arrow_up:
...Analysis/DataFrameColumns/StringDataFrameColumn.cs	69.62% <100.00%> (-1.56%)	:arrow_down:
...oft.Data.Analysis/PrimitiveDataFrameColumn.Sort.cs	87.80% <100.00%> (+0.46%)	:arrow_up:
...st/Microsoft.Data.Analysis.Tests/DataFrameTests.cs	99.90% <100.00%> (+<0.01%)	:arrow_up:
...sis/DataFrameColumns/ArrowStringDataFrameColumn.cs	63.17% <0.00%> (ø)
...nalysis/DataFrameColumns/VBufferDataFrameColumn.cs	44.44% <0.00%> (-0.16%)	:arrow_down:
...icrosoft.Data.Analysis/PrimitiveDataFrameColumn.cs	71.91% <66.66%> (-0.54%)	:arrow_down:

... and 4 files with indirect coverage changes

[asmirnov82]

cc @JakeRadMSFT