feat: support preserving field number on union field structs

[Kybxd]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.94%. Comparing base (92b805c) to head (3f3499f).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/protogen/exporter.go	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #364      +/-   ##
==========================================
+ Coverage   73.93%   73.94%   +0.01%     
==========================================
  Files          87       87              
  Lines        8662     8667       +5     
==========================================
+ Hits         6404     6409       +5     
  Misses       1698     1698              
  Partials      560      560              

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