[c++/python] Check arrow_is_string_type in _update_dataframe

[nguyenv]

• Rename ArrowAdapter::_isvar to ArrowAdapter::arrow_is_string_type and expose in public API
• In pybind11 SOMAArray._update_dataframe, check if the Arrow column's type is a string type and set the Attr's cell value to TILEDB_VAR_NUM if so

[johnkerl]

🚢
Thank you @nguyenv ! :)

[johnkerl]

Note: found on R-side debug on #3057

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.27%. Comparing base (db97b07) to head (77725d6).
Report is 85 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3111      +/-   ##
==========================================
- Coverage   88.95%   88.27%   -0.69%     
==========================================
  Files          45       47       +2     
  Lines        4293     4589     +296     
==========================================
+ Hits         3819     4051     +232     
- Misses        474      538      +64     

Flag	Coverage Δ
python	88.27% <ø> (-0.69%)	⬇️

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

Components	Coverage Δ
python_api	88.27% <ø> (-0.69%)	⬇️
libtiledbsoma	∅ <ø> (∅)