feat(storage): support listing folders as prefixes

[coryan]

Part of the work for #13561

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 6.25000% with 45 lines in your changes are missing coverage. Please review.

Project coverage is 93.27%. Comparing base (8e7f422) to head (05e5ecd).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...gle/cloud/storage/tests/object_integration_test.cc	7.14%	26 Missing ⚠️
...e/cloud/storage/examples/storage_object_samples.cc	5.55%	17 Missing ⚠️
google/cloud/storage/include_folders_as_prefixes.h	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13683      +/-   ##
==========================================
- Coverage   93.30%   93.27%   -0.03%     
==========================================
  Files        2228     2229       +1     
  Lines      193366   193414      +48     
==========================================
+ Hits       180411   180415       +4     
- Misses      12955    12999      +44     

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[scotthart]

Reviewed 7 of 7 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @coryan)