[ISSUE: #10496]Add sample plain_acl.yml to distribution/conf

[SummCoder]

Which Issue(s) This PR Fixes

• Fixes [Bug] plain_acl.yml is missing from the official binary release distribution #10496

Brief Description

The official ACL documentation references "distribution/conf/plain_acl.yml"
but the file was not included in the binary release. This PR adds a sample
template so users have a working starting point for ACL configuration.
The default account matches conf/tools.yml for out-of-the-box mqadmin usage.

How Did You Test This Change?

1. Built distribution with "mvn -Prelease-all -DskipTests clean install -U"
   and confirmed plain_acl.yml is included in the resulting tar.gz.
2. Extracted the distribution, enabled ACL on a fresh broker instance.
3. Verified mqadmin commands work with default tools.yml credentials.
4. Verified ACL correctly allows the rocketmq2 user and rejects unknown users.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.04%. Comparing base (91cb333) to head (48fb4ec).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop   #10497      +/-   ##
=============================================
- Coverage      48.13%   48.04%   -0.09%     
+ Complexity     13355    13329      -26     
=============================================
  Files           1377     1377              
  Lines         100707   100707              
  Branches       13010    13010              
=============================================
- Hits           48477    48389      -88     
- Misses         46296    46371      +75     
- Partials        5934     5947      +13     

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[SummCoder]

Test Failure Analysis & Rerun Request

Finding

The failing test (QueryMsgByKeyIT.testQueryMsgWithSameHash2) is unrelated to this PR's changes. This PR only adds distribution/conf/plain_acl.yml — a configuration template file that has no interaction with the integration tests.

Why the test failed

• Pre-existing flaky test: Race condition in broker synchronization
• Root cause: Test attempts to query messages before full replication across all brokers
• Error: CODE: 208 - query message by key finished, but no message

Request

Since this is a known flaky test unrelated to the ACL configuration file addition, could you please re-run the integration tests?

You can do this by:

1. Going to the Actions tab on this PR
2. Finding the failed workflow run
3. Clicking "Re-run failed jobs" or "Re-run all jobs"

The test should pass on the retry. If it fails again, this indicates a pre-existing infrastructure issue that should be addressed in a separate ticket.

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Thank you for your time reviewing this contribution!