[C++][GANDIVA] Add alias 'decimal' to castDecimal functions

[asfimport]

Reporter: Vinicius Souza Roque / @ViniciusSouzaRoque

PRs and other links:

• GitHub Pull Request #12833

_{Note: This issue was originally created as ARROW-16150. Please see the migration documentation for further details.}

[github-actions]

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