protogen: add field number prop

[Kybxd]

• close #218

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.68293% with 3 lines in your changes missing coverage. Please review.

Project coverage is 71.25%. Comparing base (745037b) to head (22db647). Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/protogen/exporter.go	91.89%	0 Missing and 3 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #230      +/-   ##
==========================================
+ Coverage   71.15%   71.25%   +0.09%     
==========================================
  Files          80       81       +1     
  Lines       10293    10360      +67     
==========================================
+ Hits         7324     7382      +58     
- Misses       2402     2411       +9     
  Partials      567      567              

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[wenchy]

It's too verbose to specify field number for each column.

Another solution is automation (auto keep compatibilty), when compared with the old protoconf file:

1. Auto keep field number by unique column name.
2. If new field name occured, then assign the the max field number plus 1 in the same level.

This can avoid field number overwriting for different column names.