Add CODEOWNERS file

[AlexDaines]

Adds a CODEOWNERS file per OSPO GitHub permissions guidance.

This assigns @aws/aws-sdk-dotnet-team as the default code owner for all files, enabling automatic review assignment on PRs.

https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.09%. Comparing base (fb656d3) to head (5c0093d).
⚠️ Report is 7 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1003      +/-   ##
==========================================
- Coverage   62.11%   62.09%   -0.03%     
==========================================
  Files         294      294              
  Lines       10813    10813              
  Branches     1615     1615              
==========================================
- Hits         6717     6714       -3     
- Misses       3494     3497       +3     
  Partials      602      602              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.