feat: add merge_mode for field merging

[PragmaTwice]

We add merge_mode as a template parameter of merge_field, message::merge and merge, in a API-compatible manner.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.77%. Comparing base (60f5511) to head (5e68fa4).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
include/protopuf/field.h	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage   98.87%   98.77%   -0.11%     
==========================================
  Files          29       29              
  Lines         890      896       +6     
  Branches       23       23              
==========================================
+ Hits          880      885       +5     
- Misses         10       11       +1     

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