[Feature/extensions] Rename to extensions

[mloufra]

Description

Rename method names, class names, variable names from plugin to extensions in the sdk codebase and related OpenSearch code

Equivalent PR on SDK: opensearch-project/opensearch-sdk-java#56

Issues Resolved

Rename plugin to extensions in the codebase #28

Check List

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

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[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/800/
• CommitID: c254e4e

[owaiskazi19]

These changes should be incorporated in sdk repo too otherwise the build will fail. Can you link the sdk PR in the description?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/813/
• CommitID: cdb5ba8

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/814/
• CommitID: 21e2ab3

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/828/
• CommitID: 654b7e3

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/902/
• CommitID: 5e52f00

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/1088/
• CommitID: 6f05107