Auto detect as opaque when third party struct/enum has non public field

[fzyzcjy]

Changes

Please list issues fixed by this PR here, using format "Fixes #the-issue-number".

Checklist

• An issue to be fixed by this PR is listed above.
• New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• If this PR adds/changes features, documentations (in the ./website folder) are updated.
• CI is passing. Please refer to this page to see how to solve a failed CI.

Remark for PR creator

• ./frb_internal --help shows utilities for development.
• If fzyzcjy does not reply for a few days, maybe he just did not see it, so please ping him.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.89%. Comparing base (328ac0e) to head (aa4d862).
Report is 3 commits behind head on master.

❗ Current head aa4d862 differs from pull request most recent head ca2fa41

Please upload reports for the commit ca2fa41 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2016      +/-   ##
==========================================
+ Coverage   97.88%   97.89%   +0.01%     
==========================================
  Files         414      414              
  Lines       16847    16850       +3     
==========================================
+ Hits        16490    16495       +5     
+ Misses        357      355       -2     

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