Render decimal value to string.

[karteekmurthys]

In order to support parsing decimal literals, VELOX must be able to convert decimal values into string format. This PR implements utility functions DecimalCasts::ShortDecimalToString and DecimalCasts::LongDecimalToString.

Test: Added unit tests for the above methods.

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. If you'd still like this PR merged, please comment on the PR, make sure you've addressed reviewer comments, and rebase on the latest main. Thank you for your contributions!