Add support for ModelingToolkitNeuralNets

[sebapersson]

Closes #381

This also adds supports for SciMLStructures, which will in the future enable support for #328

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 95.51821% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.19%. Comparing base (d9b0079) to head (bcb916f).

Files with missing lines	Patch %	Lines
ext/PEtabSciMLSensitivityExtension/adjoint.jl	82.75%	5 Missing ⚠️
src/common.jl	83.33%	3 Missing ⚠️
src/solve/helper.jl	92.10%	3 Missing ⚠️
src/util.jl	91.66%	3 Missing ⚠️
src/derivatives/gauss_newton.jl	66.66%	1 Missing ⚠️
src/petab_odeproblem/derivative_functions.jl	50.00%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #385      +/-   ##
==========================================
+ Coverage   94.58%   95.19%   +0.61%     
==========================================
  Files          82       83       +1     
  Lines        6938     7036      +98     
==========================================
+ Hits         6562     6698     +136     
+ Misses        376      338      -38     

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