refactor(storage): use stub for write_object

[dbolduc]

Part of the work for #2041

Implement WriteObject using the transport::Storage stub.

Similarly to the ReadObject change, we had test helpers on the builder that would no longer have access to the real implementation details (the StorageInner, hidden in the transport stub). Again, we take the request and options from the builder, and combine it with an inner from outside the builder.

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.41%. Comparing base (26cd2c9) to head (5344e89).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/storage/src/storage/perform_upload/tests.rs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3161      +/-   ##
==========================================
- Coverage   96.42%   96.41%   -0.02%     
==========================================
  Files         110      110              
  Lines        4420     4431      +11     
==========================================
+ Hits         4262     4272      +10     
- Misses        158      159       +1     

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