refactor: split IAMCredentials GenerateAccessToken()

[coryan]

We will need to parse the GenerateAccessToken() response in the external account implementation. This splits the parsing to a standalone function.

Part of the work for #5915

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This change is 

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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 (bfb006b) compared to base (2546d44).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10378      +/-   ##
==========================================
- Coverage   93.86%   93.86%   -0.01%     
==========================================
  Files        1600     1600              
  Lines      145412   145429      +17     
==========================================
+ Hits       136497   136501       +4     
- Misses       8915     8928      +13     

Impacted Files	Coverage Δ
...oud/internal/oauth2_minimal_iam_credentials_rest.h	100.00% <ø> (ø)
...ud/internal/oauth2_minimal_iam_credentials_rest.cc	72.36% <100.00%> (+4.67%)	⬆️
...ternal/oauth2_minimal_iam_credentials_rest_test.cc	100.00% <100.00%> (ø)
...ogle/cloud/spanner/testing/pick_random_instance.cc	84.21% <0.00%> (-7.90%)	⬇️
...bigtable/examples/bigtable_hello_instance_admin.cc	81.00% <0.00%> (-2.00%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.82% <0.00%> (-0.84%)	⬇️
google/cloud/bigtable/async_read_stream_test.cc	97.32% <0.00%> (-0.67%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	89.29% <0.00%> (-0.34%)	⬇️
google/cloud/storage/parallel_upload.cc	98.62% <0.00%> (+0.34%)	⬆️

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[scotthart]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @coryan)

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