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ARROW-13975: [C++] Implement decimal round

Split out of ARROW-13130. Has some overlapping changes.

Closes #11323 from lidavidm/arrow-13975

Lead-authored-by: David Li <li.davidm96@gmail.com>
Co-authored-by: Yibo Cai <yibo.cai@arm.com>
Signed-off-by: Yibo Cai <yibo.cai@arm.com>