Add neural search default processor for non OpenAI/Cohere scenario

[zane-neo]

Description

ml-commons has two default pre/post process function which are for OpenAI and Cohere and written in painless script. There's no default pre/post process function for neural search plugin text embedding case, if user want to use neural search with remote model to text embedding, user has to write complex painless script which is a heavy burden. This PR do two things:

1. Add default pre/post process function for neural search plugin text embedding and once user defines their model output to List<List> as suggested, then in the connector pre/post process can be defined to strings like: connector.pre_process.neural_search.text_embedding or connector.post_process.neural_search.text_embedding, which is pretty easy for user to use neural search text embedding.
2. Change default OpenAI & Cohere painless script to java code to make them more readable to both coder and user.

Issues Resolved

#1304

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• 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.

[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-1274-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 b48546ae89a51f303ef23ac8d605e3e343b67b34
# Push it to GitHub
git push --set-upstream origin backport/backport-1274-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-1274-to-2.x.