feat: proto conversions for BucketAccessControl

[coryan]

Fixes #4174

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This change is 

[codecov]

Codecov Report

Merging #4247 into master will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4247      +/-   ##
==========================================
- Coverage   93.39%   93.37%   -0.03%     
==========================================
  Files         862      862              
  Lines       66926    66991      +65     
==========================================
+ Hits        62508    62553      +45     
- Misses       4418     4438      +20     

Impacted Files	Coverage Δ
google/cloud/storage/bucket_access_control.h	100.00% <ø> (ø)
google/cloud/storage/internal/grpc_client.h	100.00% <ø> (ø)
google/cloud/storage/internal/grpc_client.cc	96.61% <100.00%> (+0.19%)	⬆️
...orage/internal/grpc_client_bucket_metadata_test.cc	100.00% <100.00%> (ø)
google/cloud/storage/hashing_options.h	62.50% <0.00%> (-37.50%)	⬇️
google/cloud/storage/oauth2/credentials.h	33.33% <0.00%> (-33.34%)	⬇️
google/cloud/storage/download_options.h	81.81% <0.00%> (-18.19%)	⬇️
google/cloud/storage/internal/curl_handle.h	50.00% <0.00%> (-18.19%)	⬇️
google/cloud/storage/upload_options.h	83.33% <0.00%> (-16.67%)	⬇️
...storage/benchmarks/storage_throughput_benchmark.cc	82.54% <0.00%> (-6.81%)	⬇️
... and 32 more

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