fix(storage): scopes should disable self-signed JWTs

[coryan]

The storage service does not support self-signed JWTs with scopes. With this change, self-signed JWTs will be automatically disabled when using the legacy storage::oauth2::Credentials.

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Base: 93.86% // Head: 93.86% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (57d0432) compared to base (293a4c0).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10369      +/-   ##
==========================================
- Coverage   93.86%   93.86%   -0.01%     
==========================================
  Files        1600     1600              
  Lines      145421   145412       -9     
==========================================
- Hits       136504   136492      -12     
- Misses       8917     8920       +3     

Impacted Files	Coverage Δ
...loud/internal/oauth2_service_account_credentials.h	100.00% <ø> (ø)
...oud/internal/oauth2_service_account_credentials.cc	93.03% <100.00%> (+0.07%)	⬆️
...cloud/storage/internal/unified_rest_credentials.cc	94.87% <100.00%> (-0.97%)	⬇️
...loud/storage/oauth2/service_account_credentials.cc	100.00% <100.00%> (ø)
...cloud/storage/oauth2/service_account_credentials.h	85.45% <100.00%> (-1.44%)	⬇️
...storage/oauth2/service_account_credentials_test.cc	97.72% <100.00%> (+0.01%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	97.99% <0.00%> (-0.84%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	89.29% <0.00%> (-0.67%)	⬇️
... and 3 more

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[google-cloud-cpp-bot]

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[scotthart]

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @coryan)

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google/cloud/storage/oauth2/service_account_credentials.cc line 137 at r1 (raw file):

  // Storage has more stringent requirements w.r.t. self-signed JWTs
  // than most services (which the base class
  // Disable them in the implementation class

Either some formatting or wording is off in this comment block.

Code quote:

  // Storage has more stringent requirements w.r.t. self-signed JWTs
  // than most services (which the base class
  // Disable them in the implementation class

[coryan]

Reviewable status: 4 of 6 files reviewed, all discussions resolved (waiting on @scotthart)

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google/cloud/storage/oauth2/service_account_credentials.cc line 137 at r1 (raw file):

Previously, scotthart (Scott Hart) wrote…

Either some formatting or wording is off in this comment block.

Fixed I think, PTAL.

[google-cloud-cpp-bot]

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