[Meta] Add javadocs

Description This is a meta issue to track javadoc progress for all modules that are currently missing javadocs. Once javadocs are added, we can enable missingJavadoc task as part of gradle checks, to enable strict javadoc validation.

How to reproduce <Needs update once #721 is merged>

1. Clone the repo (default main branch) and switch to remote javadoc branch (Note: javadoc branch is deprecated now)
2. Run ./gradlew missingJavadoc
3. Check the javadoc-output.txt file for failures due to missing javadocs

Logs

* Where:
Script '/Users/setiah/projects/odfe/search/gradle/missing-javadoc.gradle' line: 260

* What went wrong:
Execution failed for task ':build-tools:missingJavadoc'.
> Javadoc generation failed for :build-tools,
    Options file at: /Users/setiah/projects/odfe/search/buildSrc/build/tmp/missingJavadoc/javadoc-options.txt
    Command output at: /Users/setiah/projects/odfe/search/buildSrc/build/tmp/missingJavadoc/javadoc-output.txt <--------

Subtasks Currently most modules are failing javadoc validations due to missing javadocs.

• :benchmarks
• :build-tools
• :build-tools:reaper
• :client:benchmark
• :client:client-benchmark-noop-api-plugin
• :client:rest
• :client:rest-high-level
• :client:sniffer
• :client:test
• :client:transport
• :distribution:tools:java-version-checker
• :distribution:tools:keystore-cli
• :distribution:tools:launchers
• :distribution:tools:plugin-cli
• :doc-tools
• :example-plugins:custom-settings
• :example-plugins:custom-significance-heuristic
• :example-plugins:custom-suggester
• :example-plugins:painless-whitelist
• :example-plugins:rescore
• :example-plugins:rest-handler
• :example-plugins:script-expert-scoring
• :libs:opensearch-cli
• :libs:opensearch-core
• :libs:opensearch-dissect
• :libs:opensearch-geo
• :libs:opensearch-grok
• :libs:opensearch-nio
• :libs:opensearch-plugin-classloader
• :libs:opensearch-secure-sm
• :libs:opensearch-ssl-config
• :libs:opensearch-x-content
• :modules:aggs-matrix-stats
• :modules:analysis-common
• :modules:geo
• :modules:ingest-common
• :modules:ingest-geoip
• :modules:ingest-user-agent
• :modules:lang-expression
• :modules:lang-mustache
• :modules:lang-painless
• :modules:lang-painless:spi
• :modules:mapper-extras
• :modules:opensearch-dashboards
• :modules:parent-join
• :modules:percolator
• :modules:rank-eval
• :modules:reindex
• :modules:repository-url
• :modules:systemd
• :modules:transport-netty4
• :plugins:analysis-icu
• :plugins:analysis-kuromoji
• :plugins:analysis-nori
• :plugins:analysis-phonetic
• :plugins:analysis-smartcn
• :plugins:analysis-stempel
• :plugins:analysis-ukrainian
• :plugins:discovery-azure-classic
• :plugins:discovery-ec2
• :plugins:discovery-ec2:qa:amazon-ec2
• :plugins:discovery-gce
• :plugins:discovery-gce:qa:gce
• :plugins:ingest-attachment
• :plugins:mapper-annotated-text
• :plugins:mapper-murmur3
• :plugins:mapper-size
• :plugins:repository-azure
• :plugins:repository-gcs
• :plugins:repository-hdfs
• :plugins:repository-s3
• :plugins:store-smb
• :plugins:transport-nio
• :qa:die-with-dignity
• :qa:os
• :qa:wildfly
• :rest-api-spec
• :server (#3128, #3155, #3163)
• :test:external-modules:test-delayed-aggs
• :test:fixtures:azure-fixture
• :test:fixtures:gcs-fixture
• :test:fixtures:hdfs-fixture
• :test:fixtures:old-elasticsearch
• :test:fixtures:s3-fixture
• :test:framework
• :test:logger-usage

How to help Please add javadocs for the module you work on. Validate the module using the missingJavadoc gradle task.

For example, if you are adding javadocs for :client:rest module, you can validate your changes using command ./gradlew :client:rest:missingJavadoc command.