feat(r): Add ArrowArrayStream implementation to support keeping a dependent object in scope

[paleolimbot]

This is useful because the ADBC spec requires that the AdbcStatement must outlive the stream. GDAL's ArrowArrayStream export from OGR also specifies something like this. If the stream is getting consumed from R it's no problem; however, we want to be able to export these streams elsewhere (e.g., DuckDB, Polars) and handling it at the C level ensures that flexibility.

See also r-dbi/adbc#4

[codecov-commenter]

Codecov Report

Merging #194 (2faed29) into main (d408272) will increase coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main     #194      +/-   ##
==========================================
+ Coverage   87.74%   87.75%   +0.01%     
==========================================
  Files          59       60       +1     
  Lines        8819     8879      +60     
==========================================
+ Hits         7738     7792      +54     
- Misses       1081     1087       +6     

Impacted Files	Coverage Δ
r/src/array_stream.h	88.23% <ø> (+5.88%)	⬆️
r/src/init.c	100.00% <ø> (ø)
r/R/pointers.R	91.30% <77.77%> (-8.70%)	⬇️
r/src/pointers.c	93.02% <86.66%> (-0.84%)	⬇️
r/src/array_stream.c	85.54% <91.89%> (+5.10%)	⬆️
r/src/util.h	100.00% <100.00%> (ø)

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