Add CODEOWNERS file for team review enforcement

[cenedhryn]

Summary

Add .github/CODEOWNERS to require team review on all PRs to the develop branch.

This is part of aligning GitHub repository permissions with OSPO guidance (Python-8717).

Changes

• Added .github/CODEOWNERS with * @boto/sync-team-aws-sdk-python to ensure all PRs require approval from the Python SDK core team.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.42%. Comparing base (ee748e4) to head (6502efd).
⚠️ Report is 313 commits behind head on develop.
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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3735      +/-   ##
===========================================
- Coverage    92.59%   92.42%   -0.18%     
===========================================
  Files           68       68              
  Lines        15634    15829     +195     
===========================================
+ Hits         14477    14630     +153     
- Misses        1157     1199      +42     

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