[C++] Add cast from decimal to string and back

[asfimport]

We have Decimal128::ToString and Decimal256::ToString. I don't know how performant they are but it should be better than nothing. In addition, we have a lot of casting infrastructure already so this should just be an addition to that. There shouldn't be a lot of work required here. It will mostly be understanding how all the pieces are organized and connecting them together.

Reporter: Weston Pace / @westonpace
Assignee: Quang Hoang / @quanghgx

Related issues:

• [C++] CSV Writer: Unsupported cast from decimal to utf8 (is related to)

PRs and other links:

• GitHub Pull Request #14232

_{Note: This issue was originally created as ARROW-17042. Please see the migration documentation for further details.}

[github-actions]

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