feat(GCS+gRPC): implement To/FromProto for HmacKeyMetadata

[coryan]

Fixes #4213

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #8926 (e32c789) into main (4a8f321) will increase coverage by 0.00%.
The diff coverage is 94.59%.

@@           Coverage Diff           @@
##             main    #8926   +/-   ##
=======================================
  Coverage   93.25%   93.25%           
=======================================
  Files        1479     1481    +2     
  Lines      125259   125296   +37     
=======================================
+ Hits       116805   116841   +36     
- Misses       8454     8455    +1     

Impacted Files	Coverage Δ
google/cloud/storage/hmac_key_metadata.h	100.00% <ø> (ø)
.../storage/internal/grpc_hmac_key_metadata_parser.cc	91.66% <91.66%> (ø)
...age/internal/grpc_hmac_key_metadata_parser_test.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/subscriber_connection_test.cc	97.91% <0.00%> (+0.69%)	⬆️

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[google-cloud-cpp-bot]

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For commit: e32c7899037a610076e21233293b1810cf579e8e

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