Implement SortOptionsVariant in FieldSort

[suzuki764]

Description

Implemented SortOptionsVariant in FieldSort to make it usable with SortOptionsBuilders.

Issues Resolved

Closes #1089

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[reta]

👍 looks like a miss, thanks @suzuki764 !

[opensearch-trigger-bot]

The backport to 2.x 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-java/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/opensearch-java/backport-2.x
# Create a new branch
git switch --create backport/backport-1323-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 f9540f7829af91b740f6e67dc807a1df5d3ff497
# Push it to GitHub
git push --set-upstream origin backport/backport-1323-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/opensearch-java/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-1323-to-2.x.

[reta]

@suzuki764 could you please backport to 2.x branch manually? thank you

[suzuki764]

@reta
I have created a backport PR #1329. Please review it, thank you!