Made row_group::get_field_columns public

[ritchie46]

This function can be used when reading synchronously multiple columns from a parquet file in parallel.

[codecov]

Codecov Report

Merging #1110 (53ce452) into main (a11e258) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1110      +/-   ##
==========================================
- Coverage   81.31%   81.29%   -0.02%     
==========================================
  Files         366      366              
  Lines       35352    35352              
==========================================
- Hits        28746    28741       -5     
- Misses       6606     6611       +5     

Impacted Files	Coverage Δ
src/io/parquet/read/row_group.rs	99.40% <100.00%> (ø)
src/io/ipc/read/array/boolean.rs	89.58% <0.00%> (-8.34%)	⬇️
src/array/binary/mod.rs	88.88% <0.00%> (-1.24%)	⬇️
src/bitmap/utils/slice_iterator.rs	97.56% <0.00%> (-1.22%)	⬇️
src/array/utf8/mod.rs	91.74% <0.00%> (+0.95%)	⬆️

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