added cluster and index setting to control plan explain

[animodak7]

Description

Added index setting index.search.query_plan.explain to print query plan explain.
Default is false.

curl -X PUT "http://localhost:9200/clickbench/_settings" \
  -H "Content-Type: application/json" \
  -d '{
    "index.search.query_plan.explain": true
  }'

Added cluster setting search.query_plan.explain to print query plan explain.
Default is false.

curl -X PUT "http://localhost:9200/_settings" \                                                                                     ─╯
  -H "Content-Type: application/json" \
  -d '{
    "search.query_plan.explain": false
  }'

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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✨ Finishing touches

🧪 Generate unit tests (beta)

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[bharath-techie]

please remove ? and handle exceptions

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