[Feature/extensions] Warn future developers not to pass headers to extensions

[peternied]

Description

Warn future developers not to pass headers to extensions

Issues Resolved

• Related [Extensions] How to ensure REST Requests passed extension do not expose sensitive information? #4429

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) Note; I did not include any change log changes because I don't see a change log in this branch

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/2765/
• CommitID: e3f4352

[dbwiddis]

I think "here there be dragons" would be a more fun comment, but this one suffices.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/2844/
• CommitID: caa1da9

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/2916/
• CommitID: 1a8a26e

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/2918/
• CommitID: 039ad26

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.59%. Comparing base (14b899b) to head (039ad26).
⚠️ Report is 296 commits behind head on feature/extensions.

Additional details and impacted files

@@                   Coverage Diff                    @@
##             feature/extensions    #4430      +/-   ##
========================================================
- Coverage                 70.61%   70.59%   -0.03%     
+ Complexity                57436    57416      -20     
========================================================
  Files                      4641     4641              
  Lines                    276249   276249              
  Branches                  40384    40384              
========================================================
- Hits                     195083   195016      -67     
- Misses                    64847    64930      +83     
+ Partials                  16319    16303      -16     

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[peternied]

@opensearch-project/security What do you folks think, is this a worthwhile stopgap measure?