[Backport 2.x] Added validation for the new clusters field. (#633)

[AWSHurneyt]

Description

Manual cherry-pick of PR https://github.com/opensearch-project/common-utils/pull/633

Issues Resolved

[List any issues this PR will resolve]

Check List

• [ ] New functionality includes testing.
  • [ ] All tests pass
• [ ] New functionality has been documented.
  • [ ] New functionality has javadoc added
• [x] Commits are signed per the DCO using --signoff

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