feat: implemented MakeUserAccountCredentials

[sachinpro]

The C++ SDK function MakeServiceAccountCredentials throws Invalid ServiceAccountCredentials, the client_email field is missing on data loaded from memory for a json with authorized_user.

The PR implements MakeUserAccountCredentials, a dedicated option for authorized_user credentials.

[gemini-code-assist]

Code Review

This pull request introduces support for user account credentials by adding the MakeUserAccountCredentials factory function and the AuthorizedUserConfig class. The changes include updates to the credential visitor pattern, implementation of REST transport support, and explicit handling for gRPC transport which currently returns an unimplemented error. Feedback was provided regarding a potential issue in unified_rest_credentials.cc where the unspecified order of evaluation for function arguments could result in using a moved-from client_factory_ object.

[gemini-code-assist]

The order of evaluation for function arguments is unspecified in C++. In this statement, std::move(client_factory_) might be evaluated before client_factory_ is passed to std::make_shared, which would result in an empty or invalid factory being used to create the credentials. It is safer to create the credentials object in a separate statement to ensure the factory is copied before it is moved into the Decorate function.

      auto creds = std::make_shared<oauth2_internal::AuthorizedUserCredentials>(
          *info, cfg.options(), client_factory_);
      result = Decorate(std::move(creds), std::move(client_factory_),
                        cfg.options());

[sachinpro]

Done.

[scotthart]

/gcbrun

[codecov]

Codecov Report

❌ Patch coverage is 97.36842% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.70%. Comparing base (3852865) to head (6f551a7).

Files with missing lines	Patch %	Lines
google/cloud/internal/unified_rest_credentials.cc	80.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16090   +/-   ##
=======================================
  Coverage   92.70%   92.70%           
=======================================
  Files        2353     2353           
  Lines      218352   218428   +76     
=======================================
+ Hits       202425   202497   +72     
- Misses      15927    15931    +4     

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