fix: store sparsity pattern for symbolic backends

[gdalle]

Fixes #763 by giving a sparsity field to every sparse Jacobian and Hessian prep type.

@franckgaga wanna take it for a spin?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.88%. Comparing base (51c56e8) to head (3848862).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #764   +/-   ##
=======================================
  Coverage   97.87%   97.88%           
=======================================
  Files         128      128           
  Lines        7444     7464   +20     
=======================================
+ Hits         7286     7306   +20     
  Misses        158      158           

Flag	Coverage Δ
DI	98.95% <100.00%> (+<0.01%)	⬆️
DIT	95.20% <ø> (ø)

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[franckgaga]

@gdalle tested with AutoSymbolics and it works great, good job!

[gdalle]

Works with FastDifferentiation too, it might be interesting to compare

[franckgaga]

I will post the comparison here: JuliaControl/ModelPredictiveControl.jl#174