feat(storage): support Bucket custom placement config

[coryan]

Fixes #9135

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #9481 (bee617e) into main (3753156) will increase coverage by 0.00%.
The diff coverage is 94.82%.

@@           Coverage Diff            @@
##             main    #9481    +/-   ##
========================================
  Coverage   94.64%   94.64%            
========================================
  Files        1485     1485            
  Lines      136864   136977   +113     
========================================
+ Hits       129530   129640   +110     
- Misses       7334     7337     +3     

Impacted Files	Coverage Δ
google/cloud/storage/bucket_metadata.cc	98.24% <71.42%> (-0.57%)	⬇️
...ud/storage/internal/grpc_bucket_metadata_parser.cc	92.30% <87.50%> (-0.25%)	⬇️
...e/cloud/storage/internal/bucket_metadata_parser.cc	98.03% <92.00%> (-0.40%)	⬇️
google/cloud/storage/bucket_metadata.h	99.00% <100.00%> (+0.07%)	⬆️
google/cloud/storage/bucket_metadata_test.cc	99.41% <100.00%> (+0.02%)	⬆️
...e/cloud/storage/examples/storage_bucket_samples.cc	93.86% <100.00%> (+0.01%)	⬆️
...orage/internal/grpc_bucket_metadata_parser_test.cc	99.42% <100.00%> (+0.03%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	97.98% <0.00%> (-0.17%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	98.31% <0.00%> (+0.56%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	82.00% <0.00%> (+2.00%)	⬆️
... and 1 more

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: bee617e8c816dbf1bd6a99c722404961da44dfd0

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• Kokoro Logs (requires auth)

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