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

In classes related to Decimal types: [DecimalArray](https://github.com/apache/arrow/blob/9f13810d4407d1e2fd83a975c4eb281693e2ca9c/cpp/src/arrow/array/array_decimal.cc#L35), [DecimalBuilder](https://github.com/apache/arrow/blob/9f13810d4407d1e2fd83a975c4eb281693e2ca9c/cpp/src/arrow/array/builder_decimal.cc#L38), [DecimalType](https://github.com/apache/arrow/blob/9f13810d4407d1e2fd83a975c4eb281693e2ca9c/cpp/src/arrow/type.h#L879), 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](https://issues.apache.org/jira/browse/ARROW-10479)
#### Related issues:
- [[C++] Make Decimal representations consistent](https://github.com/apache/arrow/issues/29258) (is related to)
#### PRs and other links:
- [GitHub Pull Request #8578](https://github.com/apache/arrow/pull/8578)

_{**Note**: *This issue was originally created as [ARROW-10479](https://issues.apache.org/jira/browse/ARROW-10479). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by comparing the DecimalArray and DecimalBuilder implementations in cpp/src/arrow/array/array_decimal.cc and builder_decimal.cc with DecimalType in cpp/src/arrow/type.h. Review the related consistency issue and PR for context before deciding the shared-class design. Done means common Decimal logic is centralized and adding another bit width requires less duplicated C++ code.

Written by the indexing model from the issue text.