refactor(GCS+gRPC): use ReadAll() helper

[coryan]

This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.59%. Comparing base (8dc34f6) to head (f4c33d0).
Report is 85 commits behind head on main.

Files with missing lines	Patch %	Lines
...le/cloud/storage/internal/async/connection_impl.cc	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14557      +/-   ##
==========================================
- Coverage   93.60%   93.59%   -0.01%     
==========================================
  Files        2318     2316       -2     
  Lines      207335   206819     -516     
==========================================
- Hits       194069   193569     -500     
+ Misses      13266    13250      -16     

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[scotthart]

Reviewed 8 of 8 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @coryan)