[C++] Add support for Decimal16, Decimal32 and Decimal64

[asfimport]

It looks like arrow lacks support for decimal16, decimal32 and decimal64 types. Are there any reasons for that?

Reporter: Artem Alekseev / @fexolm

Subtasks:

• [C++][Decimal] Get rid of code duplication at Decimal

Related issues:

• [C++] Make Decimal representations consistent (is related to)

_{Note: This issue was originally created as ARROW-9404. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
See https://lists.apache.org/thread.html/r97eecb373f5ea5f1c65a6f061c75af1ef7ac460f722f4c98a5c70dc2%40%3Cdev.arrow.apache.org%3E

You are welcome to propose formalizing smaller decimal types on the mailing list. Someone would need to volunteer to do the implementation work for it, also.

[asfimport]

Artem Alekseev / @fexolm:
Thanks, @wesm. Please let me prepare the PR first, it would be useful for the sake of discussion.

[asfimport]

Ben Baumgold:
Looks like #8578 implemented this feature, but the PR seems abandoned. Would be nice to find a way to push it over the finish-line somehow so Arrow can support Decimal[16|32|64].