Avoid Boxing/Unboxing on accessing elements of VBufferDataFrameColumn

[asmirnov82]

Fixes #6864

[codecov]

Codecov Report

Merging #6865 (c77f5f7) into main (9c183fc) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #6865      +/-   ##
==========================================
- Coverage   69.64%   69.63%   -0.01%     
==========================================
  Files        1237     1237              
  Lines      247636   247629       -7     
  Branches    25431    25430       -1     
==========================================
- Hits       172464   172437      -27     
- Misses      68561    68578      +17     
- Partials     6611     6614       +3     

Flag	Coverage Δ
Debug	69.63% <100.00%> (-0.01%)	⬇️
production	64.17% <100.00%> (-0.02%)	⬇️
test	88.89% <ø> (ø)

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Files	Coverage Δ
...icrosoft.Data.Analysis/PrimitiveDataFrameColumn.cs	73.56% <100.00%> (+0.12%)	⬆️

... and 6 files with indirect coverage changes