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 94.44444% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.38%. Comparing base (8a20fee) to head (4c6f86e).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
ext/LinearMPCext.jl	94.44%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #315   +/-   ##
=======================================
  Coverage   98.37%   98.38%           
=======================================
  Files          29       29           
  Lines        5369     5388   +19     
=======================================
+ Hits         5282     5301   +19     
  Misses         87       87           

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