cleanup(GCS+gRPC): move HmacKeyMetadata helpers to namespace

[coryan]

Remove the HmacKeyMetadataParser struct, which only had static member functions. Move the functions to the storage_internal namespace.

Part of the work for #8929

---

This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 29343547eb4e4122f34e9b1749dcc5334a9be469

• 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% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (2934354) compared to base (959ae37).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9955      +/-   ##
==========================================
- Coverage   94.21%   94.21%   -0.01%     
==========================================
  Files        1497     1497              
  Lines      140436   140436              
==========================================
- Hits       132312   132311       -1     
- Misses       8124     8125       +1     

Impacted Files	Coverage Δ
google/cloud/storage/internal/grpc_client.cc	98.75% <100.00%> (ø)
.../storage/internal/grpc_hmac_key_metadata_parser.cc	92.30% <100.00%> (ø)
...age/internal/grpc_hmac_key_metadata_parser_test.cc	100.00% <100.00%> (ø)
...d/storage/internal/grpc_hmac_key_request_parser.cc	85.71% <100.00%> (ø)
...le/cloud/internal/default_completion_queue_impl.cc	96.59% <0.00%> (-0.57%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.98% <0.00%> (-0.51%)	⬇️
google/cloud/pubsub/samples/samples.cc	90.70% <0.00%> (-0.08%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	89.29% <0.00%> (+1.00%)	⬆️
...integration_tests/schema_admin_integration_test.cc	100.00% <0.00%> (+1.11%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.