fix(storage): cache all credential types

[coryan]

Applications may call AuthorizedHeader() and that can generate calls to the GCE metadata service or Google's Security Token Service.

Fixes #11442

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This change is 

[codecov]

Codecov Report

Patch coverage: 98.03% and no project coverage change.

Comparison is base (90a6882) 93.78% compared to head (f95e1b1) 93.78%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #11447   +/-   ##
=======================================
  Coverage   93.78%   93.78%           
=======================================
  Files        1805     1805           
  Lines      162337   162431   +94     
=======================================
+ Hits       152244   152341   +97     
+ Misses      10093    10090    -3     

Impacted Files	Coverage Δ
...cloud/storage/oauth2/authorized_user_credentials.h	92.85% <66.66%> (+0.54%)	⬆️
.../cloud/storage/oauth2/compute_engine_credentials.h	96.66% <80.00%> (+0.11%)	⬆️
...loud/storage/oauth2/authorized_user_credentials.cc	96.00% <100.00%> (+0.44%)	⬆️
...storage/oauth2/authorized_user_credentials_test.cc	98.00% <100.00%> (+0.36%)	⬆️
...cloud/storage/oauth2/compute_engine_credentials.cc	100.00% <100.00%> (ø)
.../storage/oauth2/compute_engine_credentials_test.cc	100.00% <100.00%> (ø)

... and 7 files with indirect coverage changes

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