feat(storage): migrate sync client to unified checksum options

[v-pratap]

Migrates the Sync client 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 #16261 and includes its commits. It should be merged after #16261.

[gemini-code-assist]

Code Review

This pull request introduces UploadChecksumValidationOption and DownloadChecksumValidationOption using a new ChecksumAlgorithm enum to replace several deprecated checksum options and classes. It updates internal hash functions, validators, and tests to support these new options. The review feedback highlights that using the deprecated types in hash_function.cc, hash_validator.cc, and object_requests.cc will trigger -Wdeprecated-declarations compiler warnings, suggesting the inclusion of deprecation warning disable headers. Additionally, the reviewer recommends refactoring duplicated fallback logic for resolving checksum settings into a shared helper function to align with the repository style guide.

[codecov]

Codecov Report

❌ Patch coverage is 98.43750% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.27%. Comparing base (fd956fb) to head (c7928ae).

Files with missing lines	Patch %	Lines
google/cloud/storage/client_object_test.cc	96.72%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16264      +/-   ##
==========================================
- Coverage   92.27%   92.27%   -0.01%     
==========================================
  Files        2218     2219       +1     
  Lines      206654   206686      +32     
==========================================
+ Hits       190699   190720      +21     
- Misses      15955    15966      +11     

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