feat(storage): support object retention

[coryan]

Implement support for object retention. This introduces a new object
metadata field. Any patch or update operation that changes this field
requires an extra option (OverrideUnlockedRetention).

Part of the work for #12994

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 98.31933% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.19%. Comparing base (b514243) to head (b2c1d55).

❗ Current head b2c1d55 differs from pull request most recent head 08c1e9a

Please upload reports for the commit 08c1e9a to get more accurate results.

Files	Patch %	Lines
google/cloud/storage/override_unlocked_retention.h	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #14351    +/-   ##
========================================
  Coverage   93.19%   93.19%            
========================================
  Files        2187     2191     +4     
  Lines      192250   192367   +117     
========================================
+ Hits       179160   179272   +112     
- Misses      13090    13095     +5     

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[bajajneha27]

LGTM except for two tiny comments.

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• google/cloud/storage/examples/storage_bucket_object_retention_samples.cc:26, tag storage_create_bucket_with_object_retention
• google/cloud/storage/examples/storage_object_retention_samples.cc:95, tag storage_set_object_retention_policy

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[coryan]

Thanks!