chore(storage): use designated initializers for client.cc/h files

[Pulkit0110]

Formatted storage client.cc and client.h files to use designated initializers as per https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-designated-initializers.html#modernize-use-designated-initializers.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.90%. Comparing base (7d87eb6) to head (43b6edd).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/storage/client.cc	94.44%	1 Missing ⚠️
google/cloud/storage/client.h	83.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15106   +/-   ##
=======================================
  Coverage   92.90%   92.90%           
=======================================
  Files        2360     2360           
  Lines      211708   211712    +4     
=======================================
+ Hits       196685   196699   +14     
+ Misses      15023    15013   -10     

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[scotthart]

Thanks for the PR, but unfortunately we won't be able to merge these changes at this time. Designated initializers are a c++20 feature (https://en.cppreference.com/w/cpp/language/aggregate_initialization). While compilers some times implement features prior to their standardization, in order to maintain portability, we cannot use them ahead of time.

The silver lining is that this PR has brought to our attention that we do not have any CI builds that enable -pedantic which would have caught this. We will be adding -pdenatic to the clng-tidy builds in order to prevent this from happening in the future.