[C++][Compute] Support casts to and from Decimal256 type.

[asfimport]

Reporter: Micah Kornfield / @emkornfield
Assignee: David Li / @lidavidm

PRs and other links:

• GitHub Pull Request #9751

_{Note: This issue was originally created as ARROW-10606. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 9751
#9751