Fetch index settings asynchronously

[msfroh]

In some cases, like using ./gradlew run, I've run into issues where OpenSearch crashes, complaining that we're making a blocking call on a listener thread, because we fetch index settings to see if a result transformer has been configured for the current index.

I'm kind of surprised that we haven't run into this in production use-cases, but it may just be because assertions are not enabled in production. Regardless, blocking calls on listener threads are a bad idea, since that's how you run out of listener threads.

This change makes the index settings call asynchronous and chains the remaining logic off of that.

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[sejli]

LGTM! I did notice that some of the formatting was changed on some files, particularly ConfigurationUtils.java. This repo should probably include the Spotless Gradle plugin for a formatting check.

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-135-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 fdadd8cb8c70d7732650b52614756d8fa2569a97
# Push it to GitHub
git push --set-upstream origin backport/backport-135-to-2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-135-to-2.x.