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

In classes related to Decimal types: DecimalArray, DecimalBuilder, DecimalType, etc, there is a lot of code duplications between decimals with different bit widths.

The proposal here is to create base classes for them, that will contain common logic for decimals of all bit widths.

These changes should simplify adding of new width decimals to C++ part.

Reporter: Dmitry Chigarev

Related issues:

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

PRs and other links:

• GitHub Pull Request #8578

Note: This issue was originally created as ARROW-10479. Please see the migration documentation for further details.