Add support for serializing Spark's DecimalType

[engechas]

Description

Adds support for serializing Spark's DecimalType in FlintDataType

Related Issues

List any issues this PR will resolve, e.g. Resolves [...].

Check List

• Updated documentation (docs/ppl-lang/README.md)
• Implemented unit tests
• Implemented tests for combination with other commands
• New added source code should include a copyright header
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[engechas]

Could we backport this change to 0.6?

[dai-chen]

Thanks for the changes!

[opensearch-trigger-bot]

The backport to 0.6 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/opensearch-spark/backport-0.6 0.6
# Navigate to the new working tree
pushd ../.worktrees/opensearch-spark/backport-0.6
# Create a new branch
git switch --create backport/backport-947-to-0.6
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 0943f1fbe893349688aaa84c1d1fcab69156d264
# Push it to GitHub
git push --set-upstream origin backport/backport-947-to-0.6
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/opensearch-spark/backport-0.6

Then, create a pull request where the base branch is 0.6 and the compare/head branch is backport/backport-947-to-0.6.

[noCharger]

@engechas could you backport to 0.6 manually?

[engechas]

Created #962 to manually backport to 0.6