Migrating from checkstyle to spotless for formatting

[VachaShah]

Description

Adding spotless support in this repo in place of checkstyle so that we can get features for checking and applying the formatting using spotless. The rules used here are from the OpenSearch repo. spotlessJavaCheck is now part of the build task making it easier for developers to format their changes.

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

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[reta]

Sorry @VachaShah , it is impossible to review the PRs as large as those: what we did for security & core - migrate per packages, in chunks of pull requests of no more than 300 files, it adds up some work but at least is feasible to go through the changes.

[VachaShah]

Sorry @VachaShah , it is impossible to review the PRs as large as those: what we did for security & core - migrate per packages, in chunks of pull requests of no more than 300 files, it adds up some work but at least is feasible to go through the changes.

Yeah I thought about this after opening the PR 🙈 Let me break this down in multiple PRs without breaking the builds.

[dblock]

I wouldn't mind one massive PR as long as it's clear that the whole change is a couple of files and a separate "ran ./gradlew spotless:apply" kind of thing and all tests pass. The other option is to find a way where spotless is enabled, but some rules are disabled, and make PRs that enable them one-by-one.

[VachaShah]

Thank you @dblock! I started dividing this, first PR is #648 which includes support for spotless and enabling spotless for some modules. The subsequent PRs will enable for other folders.