feat(storage): Migrate async client to unified checksum options

[v-pratap]

Migrates the Async client (connection_impl.cc, object_descriptor_impl.cc) to use the newly introduced unified checksum options. Also includes the necessary fallback logic to support users who are still using the deprecated options.

Note: This PR is stacked on top of #16260 and includes its commit. It should be merged after #16260.

[gemini-code-assist]

Code Review

This pull request introduces UploadChecksumValidationOption and DownloadChecksumValidationOption to configure checksum algorithms for uploads and downloads, deprecating the older EnableCrc32cValidationOption and EnableMD5ValidationOption. It updates the internal connection and reader implementations to respect these new options with fallback support, adds corresponding unit and integration tests, and provides a code sample. The reviewer feedback highlights that the fallback logic for download checksum validation is duplicated three times across non-test files and suggests refactoring it into a shared helper function in accordance with the repository style guide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.27%. Comparing base (e493b2d) to head (16ac115).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16261      +/-   ##
==========================================
- Coverage   92.27%   92.27%   -0.01%     
==========================================
  Files        2214     2215       +1     
  Lines      206505   206544      +39     
==========================================
+ Hits       190556   190586      +30     
- Misses      15949    15958       +9     

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