feat(storage): easier mocks for HmacKeyMetadata

[coryan]

Add modifiers to make mocking easier. Remove unneeded friend declarations after this change.

Part of the work for #8929

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This change is 

[google-cloud-cpp-bot]

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For commit: 8b1dd6c99d479dab6ed86fd4243b86413fe32654

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[codecov]

Codecov Report

Base: 94.20% // Head: 94.20% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (8b1dd6c) compared to base (e8fb790).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9949   +/-   ##
=======================================
  Coverage   94.20%   94.20%           
=======================================
  Files        1497     1497           
  Lines      140407   140428   +21     
=======================================
+ Hits       132273   132297   +24     
+ Misses       8134     8131    -3     

Impacted Files	Coverage Δ
google/cloud/storage/hmac_key_metadata.h	100.00% <100.00%> (ø)
.../storage/internal/grpc_hmac_key_metadata_parser.cc	92.30% <100.00%> (ø)
...cloud/storage/internal/hmac_key_metadata_parser.cc	100.00% <100.00%> (ø)
...le/cloud/storage/internal/curl_download_request.cc	88.29% <0.00%> (-1.01%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	96.59% <0.00%> (-0.57%)	⬇️
google/cloud/storage/parallel_upload.cc	98.26% <0.00%> (-0.35%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.98% <0.00%> (+0.16%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.00% <0.00%> (+2.00%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️
...e/cloud/spanner/testing/cleanup_stale_instances.cc	75.40% <0.00%> (+6.55%)	⬆️

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