Updated dev guide with the correct GET route.

[dblock]

Description

Fixed up dev guide, notably using the correct GET route for the hello world request.
Added a JSON configuration file to avoid POSTing the entire JSON body when registering the extension.

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[owaiskazi19]

@dblock we have spotless for MD files as well 😛.
./gradlew :spotlessApply to fix the spotless failures.

[opensearch-trigger-bot]

The backport to 1.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-sdk-java/backport-1.x 1.x
# Navigate to the new working tree
pushd ../.worktrees/opensearch-sdk-java/backport-1.x
# Create a new branch
git switch --create backport/backport-944-to-1.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 ee55c5d36854e7b4d8c43afc4dc6b0c0212ccf92
# Push it to GitHub
git push --set-upstream origin backport/backport-944-to-1.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/opensearch-sdk-java/backport-1.x

Then, create a pull request where the base branch is 1.x and the compare/head branch is backport/backport-944-to-1.x.