Add support for string vectors to DataFrame

[JakeRadMSFT]

This PR adds support for string vector/"ReadOnlyMemory" vectors.

Relevant issue: #5872

Building on: #6409

[codecov]

Codecov Report

Merging #6628 (f7b8346) into main (c0220f7) will decrease coverage by 0.01%.
The diff coverage is 55.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6628      +/-   ##
==========================================
- Coverage   68.38%   68.37%   -0.01%     
==========================================
  Files        1176     1176              
  Lines      248210   248218       +8     
  Branches    25915    25917       +2     
==========================================
- Hits       169743   169731      -12     
- Misses      71689    71711      +22     
+ Partials     6778     6776       -2     

Flag	Coverage Δ
Debug	68.37% <55.55%> (-0.01%)	⬇️
production	62.75% <33.33%> (-0.01%)	⬇️
test	88.87% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
.../Microsoft.Data.Analysis/VBufferDataFrameColumn.cs	46.97% <0.00%> (-0.51%)	⬇️
src/Microsoft.Data.Analysis/IDataView.Extension.cs	91.89% <66.66%> (+0.16%)	⬆️
...oft.Data.Analysis.Tests/DataFrameIDataViewTests.cs	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes