feat(cmd/gen): support mapping _grpc.pb.go files to different Go packages from .pb.go

[qianlongzt]

#8233

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.20%. Comparing base (732f3f3) to head (c0bb495).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8245      +/-   ##
==========================================
+ Coverage   82.09%   82.20%   +0.10%     
==========================================
  Files         412      412              
  Lines       40491    40491              
==========================================
+ Hits        33242    33286      +44     
+ Misses       5876     5842      -34     
+ Partials     1373     1363      -10     

see 25 files with indirect coverage changes

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[arjan-bal]

Thanks for sharing the code, I'll close the PR for now as I don't want it to be reviewed yet. Once we agree on the approach in the issue, we will re-open this.