Move getMetadataFields to IndexService

[cwperks]

Description

Reworks #13819 to move the method getMetadataFields from the MapperService to IndexService. During the 2.15.0 release testing it was seen that the mapperService returned from indexService.mapperService() can be null in some instances like the closing of an index (See needsMapperService and the IndexService constructor).

NPE was seen here: https://github.com/opensearch-project/security/blob/main/src/main/java/org/opensearch/security/configuration/SecurityFlsDlsIndexSearcherWrapper.java#L65

Related Issues

Related to opensearch-project/security#4475

Check List

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

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❌ Gradle check result for 4e21c9d: FAILURE

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❌ Gradle check result for b71d1ed: FAILURE

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[github-actions]

❌ Gradle check result for b71d1ed: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?