cleanup(GCS+gRPC): move SignBlob request helpers

[coryan]

Despite the branch name, this is only part of the work for #8929

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 3258bb9c1739c9e1061d6022dec25127b06d7a8f

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Base: 94.21% // Head: 94.21% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (3258bb9) compared to base (d25d981).
Patch coverage: 90.90% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9962   +/-   ##
=======================================
  Coverage   94.21%   94.21%           
=======================================
  Files        1497     1497           
  Lines      140499   140500    +1     
=======================================
+ Hits       132373   132374    +1     
  Misses       8126     8126           

Impacted Files	Coverage Δ
google/cloud/storage/internal/grpc_client.cc	98.75% <50.00%> (ø)
.../storage/internal/grpc_sign_blob_request_parser.cc	93.33% <100.00%> (ø)
...age/internal/grpc_sign_blob_request_parser_test.cc	100.00% <100.00%> (ø)
...cloud/pubsub/internal/subscription_session_test.cc	97.82% <0.00%> (-0.34%)	⬇️
google/cloud/storage/parallel_upload.cc	98.61% <0.00%> (+0.34%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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