[r/c++] Rcpp/libtiledbsoma integration: SOMASparseNDArray read path

[johnkerl]

As described on #1632 @nguyenv has (a) been expanding C++ class coverage; (b) been integrating them into Python -- creating the former, and making the latter a thinner layer.

For #2406 we want to continue this pattern.

@eddelbuettel and @nguyenv can collaborate -- suggested 2 of 6 is SOMASparseNDArray read path