Configure spotless for asynchronous-search

[cwperks]

Description

This PR adds spotless to this plugin which helps with consistent styling across components. Took the same config that's present from other component repos. Example JS: https://github.com/opensearch-project/job-scheduler/pull/290

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [ ] Commits are signed per the DCO using --signoff.
• [ ] Public documentation issue/PR created.

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