[WIP] Java Plugin Experiments

[andrewvc]

https://github.com/andrewvc/CowObserver is a demo plugin. checkout its source as a sibling to the Logstash folder. Build the plugin with ./gradlew devInstall. You can build a zip of it with ./gradlew zip.

this is very WIP

[mergify]

This pull request does not have a backport label. Could you fix it @andrewvc? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.

[mergify]

This pull request is now in conflicts. Could you fix it @andrewvc? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b java-plugins upstream/java-plugins
git merge upstream/main
git push upstream java-plugins