feat(credentials): add support for creating ComputeEngine credentials explicitly

[scotthart]

fixes #15673

[codecov]

Codecov Report

❌ Patch coverage is 71.87500% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.94%. Comparing base (45fe5af) to head (83cd9ea).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...cloud/storage/internal/unified_rest_credentials.cc	0.00%	6 Missing ⚠️
...oud/internal/grpc_compute_engine_authentication.cc	70.58%	5 Missing ⚠️
google/cloud/testing_util/credentials.h	0.00%	4 Missing ⚠️
google/cloud/internal/unified_grpc_credentials.cc	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15789      +/-   ##
==========================================
- Coverage   92.94%   92.94%   -0.01%     
==========================================
  Files        2454     2457       +3     
  Lines      226878   226942      +64     
==========================================
+ Hits       210879   210926      +47     
- Misses      15999    16016      +17     

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[diegomarquezp]

LGTM code-wise, but I'd like to understand how does credential parsing work in cpp. In java, we parse a stream or JSON containing expected fields per credential type and validate their existence, but I'm not sure if the same mechanism exists in cpp auth.