cleanup(mixin): refactor mixin utils to use HttpRule

[cuiy0006]

Refactor mixin utils to use HttpRule as http overrides.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 97.67442% with 3 lines in your changes missing coverage. Please review.

Project coverage is 93.50%. Comparing base (fbc2aa6) to head (ad44e99).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
generator/internal/http_option_utils.cc	90.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14723      +/-   ##
==========================================
- Coverage   93.50%   93.50%   -0.01%     
==========================================
  Files        2323     2323              
  Lines      207947   207939       -8     
==========================================
- Hits       194440   194432       -8     
  Misses      13507    13507              

Flag	Coverage Δ
	93.50% <97.67%> (?)

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[dbolduc]

Looks great. Thanks!