feat(GCS+gRPC): implement BucketBilling parser

[coryan]

Fixes #4164

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #7765 (f22c666) into main (67dce82) will increase coverage by 0.00%.
The diff coverage is 94.73%.

@@           Coverage Diff           @@
##             main    #7765   +/-   ##
=======================================
  Coverage   95.15%   95.15%           
=======================================
  Files        1273     1275    +2     
  Lines      114693   114712   +19     
=======================================
+ Hits       109135   109157   +22     
+ Misses       5558     5555    -3     

Impacted Files	Coverage Δ
...ud/storage/internal/grpc_bucket_metadata_parser.cc	88.88% <88.88%> (ø)
...orage/internal/grpc_bucket_metadata_parser_test.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/samples/samples.cc	92.10% <0.00%> (+0.07%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️
...ud/spanner/integration_tests/client_stress_test.cc	86.18% <0.00%> (+0.65%)	⬆️
...integration_tests/schema_admin_integration_test.cc	100.00% <0.00%> (+1.08%)	⬆️

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[dbolduc]

s/2019/2021/

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: f22c66658bbb0136b783c98af504b6f1c3f40e78

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