feat(storage): take async client out of experimental

[bajajneha27]

This change is 

[codecov]

Codecov Report

❌ Patch coverage is 96.57388% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.96%. Comparing base (fa35817) to head (0dc643b).

Files with missing lines	Patch %	Lines
...le/cloud/storage/examples/storage_async_samples.cc	83.01%	9 Missing ⚠️
...le/cloud/storage/internal/async/connection_impl.cc	87.17%	5 Missing ⚠️
...rage/internal/async/object_descriptor_impl_test.cc	97.29%	1 Missing ⚠️
google/cloud/storage/testing/mock_resume_policy.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15573      +/-   ##
==========================================
- Coverage   92.96%   92.96%   -0.01%     
==========================================
  Files        2460     2460              
  Lines      228931   228901      -30     
==========================================
- Hits       212826   212789      -37     
- Misses      16105    16112       +7     

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[shubham-up-47]

Please search experimental and gcs_ex keywords in your local repo, so that we don't miss these words in any file.

Please fix the code coverage which are affected by these changes.

[scotthart]

Can we add something to CHANGELOG.md that calls out the promotion of these features from experimental to GA?