The PR fixes issue: #15140 which says to provide a function to convert storage_experimental::AsyncReader into a single future<StatusOr<ReadPayload>>. The functionality is already there in storage_experimental::ReadAll() method.

This PR adds the method to a public header i.e. async client and updates the documentation.

This change is 

Codecov Report

❌ Patch coverage is 99.08257% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.02%. Comparing base (c7d96c9) to head (7fd64bc).
⚠️ Report is 274 commits behind head on main.

@@            Coverage Diff             @@
##             main   #15303      +/-   ##
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- Coverage   93.02%   93.02%   -0.01%     
==========================================
  Files        2403     2403              
  Lines      219424   219533     +109     
==========================================
+ Hits       204128   204228     +100     
- Misses      15296    15305       +9     

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