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.