[Backport 2.x] [BUG] Zstd new codec is a breaking change for kNN plugin: move custom-codecs from sandbox/modules to sandbox/plugins (so it won't be installed automatically)

[opensearch-trigger-bot]

Backport c7d6cf5 from #7014.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.backwards.MixedClusterClientYamlTestSuiteIT.test {p0=pit/10_basic/Delete all}

• URL: https://build.ci.opensearch.org/job/gradle-check/13580/
• CommitID: bd9f369
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.43%. Comparing base (28dff10) to head (bd9f369).
⚠️ Report is 2304 commits behind head on 2.x.

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x    #7019   +/-   ##
=========================================
  Coverage     70.43%   70.43%           
- Complexity    59503    59545   +42     
=========================================
  Files          4822     4822           
  Lines        285866   285866           
  Branches      41548    41548           
=========================================
+ Hits         201349   201360   +11     
- Misses        67663    67683   +20     
+ Partials      16854    16823   -31     

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