Define multiple keys for type conversion

[wanghd89]

Description

Allow defining multiple keys in the convert_entry_type processor.

Note: existing key still works. But only One Of (key or Keys) is required.
If all of them defined or Non of them defined, will throw exception.

eg:

processor:
    - convert_entry_type:
         keys: ["key1", "key2"]
         type: "integer"

converts
{"message":"value","key1":"123", "key2":"456"}
to
{"message":"value","key1":123, "key2":456}

Issues Resolved

Resolve #2823

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added
• Commits are signed with a real name per the DCO

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.

[asifsmohammed]

Changes looks good, just missing the documentation

[wanghd89]

Changes looks good, just missing the documentation

Ok, updated README

[opensearch-trigger-bot]

The backport to 2.3 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-2.3 2.3
# Navigate to the new working tree
cd .worktrees/backport-2.3
# Create a new branch
git switch --create backport/backport-2934-to-2.3
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 d93ead9f39588a3fb5d7c0d20df11a8875070fce
# Push it to GitHub
git push --set-upstream origin backport/backport-2934-to-2.3
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.3

Then, create a pull request where the base branch is 2.3 and the compare/head branch is backport/backport-2934-to-2.3.