arrow

ARROW-17995: [C++] Fix json decimals not being rescaled based on the explicit schema (#14380)

arrow::json::DecimalConverter::Convert() currently read the decimal values using the parsed precision and scale. This produces wrong results if the parsed scale doesn't match the output scale (specified by explicit schema).

More details on how to reproduce the issue are in the JIRA description. This patch fixes json::DecimalConverter::Convert() to rescale the values based on the output scale. Unit tests are added as well.

Lead-authored-by: stiga-huang <huangquanlong@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>