Support WithBatching in GCP

[AlCutter]

This PR enables support of WithBatching in the GCP storage.

Toward #23

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 32.19%. Comparing base (46ec9c2) to head (cc34fad).
Report is 13 commits behind head on main.

Files	Patch %	Lines
cmd/example-gcp/main.go	0.00%	4 Missing ⚠️
log.go	0.00%	2 Missing ⚠️
storage/gcp/gcp.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
- Coverage   35.80%   32.19%   -3.62%     
==========================================
  Files          16       16              
  Lines        1363     1491     +128     
==========================================
- Hits          488      480       -8     
- Misses        801      939     +138     
+ Partials       74       72       -2     

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