blob/gcsblob: allow providing a storage.Client directly (e.g., to use a gRPC-based client)

[vangent]

Fixes #3615.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.88%. Comparing base (3690eda) to head (e6dd5b1).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
blob/gcsblob/gcsblob.go	50.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3616      +/-   ##
==========================================
- Coverage   69.91%   69.88%   -0.04%     
==========================================
  Files         114      114              
  Lines       14763    14767       +4     
==========================================
- Hits        10322    10320       -2     
- Misses       3705     3710       +5     
- Partials      736      737       +1     

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