[Extensions] Only store names of extension transport actions

[dbwiddis]

Companion PR: opensearch-project/opensearch-sdk-java#524
(PRs must be merged together, this one first.)

Description

o.o.extensions.RegisterTransportActionsRequest was incorrectly attempting to store a map with a compiled class available only to the SDK and not to OpenSearch.

This PR changes the request to track only the set of action names (exactly equivalent to the .keySet() of the prior map implementation), which was all that was being used by the handler anyway.

The SDK will maintain a map of name to actual compiled/executable action.

Issues Resolved

SDK #484

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
  - [ ] Commit changes are listed out in CHANGELOG.md file (See: Changelog) (No change log, bug fix to prerelease feature.)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/11992/
• CommitID: deabf75
  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?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/11996/
• CommitID: ffd69d2
  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?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/11997/
• CommitID: dde5764
  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?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/12000/
• CommitID: a7aa4f2
  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?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.indices.replication.SegmentReplicationAllocationIT.testAllocationWithDisruption
      1 org.opensearch.cluster.allocation.AwarenessAllocationIT.testThreeZoneOneReplicaWithForceZoneValueAndLoadAwareness

• URL: https://build.ci.opensearch.org/job/gradle-check/12017/
• CommitID: 985e11a
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.