Fix DataFrame Saving csv with VBufferDataFrameColumn

[asmirnov82]

Fixed saving DataFrame with VBufferDataFrame column into csv file. Loading VBufferDataFrame from csv is out of scope of this PR. Currently it's load as a string column. Correct reading of csv requires redesign of the DataFrame to support save/load of user defined types (part of #6859)

Fixes #6858

[codecov]

Codecov Report

Merging #6860 (a981605) into main (64d7ebd) will increase coverage by 0.00%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #6860   +/-   ##
=======================================
  Coverage   69.63%   69.63%           
=======================================
  Files        1237     1237           
  Lines      247617   247636   +19     
  Branches    25436    25431    -5     
=======================================
+ Hits       172429   172443   +14     
- Misses      68577    68578    +1     
- Partials     6611     6615    +4     

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

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

Files	Coverage Δ
src/Microsoft.Data.Analysis/DataFrame.IO.cs	83.12% <100.00%> (-0.14%)	⬇️
...icrosoft.Data.Analysis/PrimitiveDataFrameColumn.cs	73.44% <100.00%> (ø)
src/Microsoft.ML.DataView/VBuffer.cs	94.50% <100.00%> (+0.06%)	⬆️
...Microsoft.Data.Analysis.Tests/DataFrame.IOTests.cs	99.04% <100.00%> (+0.01%)	⬆️

... and 5 files with indirect coverage changes