[c++] Performant DataFrame.shape

[johnkerl]

Issue and/or context: This follows on #2944 for issue #2407 / [sc-51048]. Here we implement shape for SOMA DataFrame, including its variant-indexed flavors.

Changes:

While SOMASparseNDArray and SOMADenseNDArray must always and only have int64 dims within the SOMA data model, SOMADataFrame is different. The default behavior -- which almost everyone uses -- has a single soma_joinid dim which is indeed of type int64. (Also note that exp.obs.shape does exist within TileDB-SOMA-Py, and people do call it.) However, the spec only requires that soma_joinid exist as a dim or an attr: it can be a dim along with others, or it can not be a dim at all. Here we do the right thing, without and with current-domain support, to produce the user-expected exp.obs.shape.

Notes for Reviewer:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.02%. Comparing base (ded1e42) to head (b2ef153).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2916      +/-   ##
==========================================
+ Coverage   89.87%   90.02%   +0.15%     
==========================================
  Files          38       38              
  Lines        3999     3999              
==========================================
+ Hits         3594     3600       +6     
+ Misses        405      399       -6     

Flag	Coverage Δ
python	90.02% <ø> (+0.15%)	⬆️

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

Components	Coverage Δ
python_api	90.02% <ø> (+0.15%)	⬆️
libtiledbsoma	∅ <ø> (∅)