feat(storage): Support read_all in the async client

[shubham-up-47]

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[bot]]

Codecov Report

:x: Patch coverage is 99.08257% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 93.02%. Comparing base (c7d96c9) to head (7fd64bc). :warning: Report is 65 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/storage/async/client.cc	94.44%	1 Missing :warning:

Additional details and impacted files

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

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[shubham-up-47]

/gcbrun

[shubham-up-47]

/gcbrun

[shubham-up-47]

/gcbrun