[GRPC] Rename package org.opensearch.plugin.transport.grpc to org.opensearch.transport.grpc

[karenyrx]

Description

Now that transport-grpc has been converted to a module rather than a plugin, remove all references of a 'plugin'.

1. Rename the package from org.opensearch.plugin.transport.grpc to org.opensearch.transport.grpc
2. Remove the plugin/ folder from all paths under modules/transport-grpc, e.g. move files from modules/transport-grpc/src/main/java/org/opensearch/plugin/transport/grpc to modules/transport-grpc/src/main/java/org/opensearch/transport/grpc
3. Fix some minor comments still referencing 'plugin' as well.

Related Issues

Related to #18893

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for cb5cf29: FAILURE

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 result for 8421a5d: UNSTABLE

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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.82%. Comparing base (9e49930) to head (8421a5d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #18923      +/-   ##
============================================
+ Coverage     72.74%   72.82%   +0.08%     
- Complexity    68840    68867      +27     
============================================
  Files          5601     5601              
  Lines        316394   316394              
  Branches      45872    45872              
============================================
+ Hits         230155   230409     +254     
+ Misses        67599    67313     -286     
- Partials      18640    18672      +32     

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