added: C codegen support for LinMPC custom linear inequality constraints

[franckgaga]

Oopsie, forgot to merge the branch with the C codegen support 🙄

[codecov-commenter]

Codecov Report

❌ Patch coverage is 61.11111% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.16%. Comparing base (8a20fee) to head (b3f236f).

Files with missing lines	Patch %	Lines
ext/LinearMPCext.jl	61.11%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #315      +/-   ##
==========================================
- Coverage   98.37%   98.16%   -0.22%     
==========================================
  Files          29       29              
  Lines        5369     5388      +19     
==========================================
+ Hits         5282     5289       +7     
- Misses         87       99      +12     

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