schemas: add from_dict to RollingResistance and Gamma

[elise-chin]

To simplify the process of creating instances from a dictionary, instead of manually extracting each attribute and passing them to the class constructor.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.02%. Comparing base (66adf79) to head (d1c804f).
Report is 1 commits behind head on dev.

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@@             Coverage Diff              @@
##                dev    #8865      +/-   ##
============================================
- Coverage     37.02%   37.02%   -0.01%     
  Complexity     2211     2211              
============================================
  Files          1255     1255              
  Lines        114227   114227              
  Branches       3182     3182              
============================================
- Hits          42297    42292       -5     
- Misses        70039    70044       +5     
  Partials       1891     1891              

Flag	Coverage Δ
core	74.84% <ø> (ø)
editoast	72.44% <ø> (-0.02%)	⬇️
front	14.89% <ø> (ø)
gateway	2.20% <ø> (ø)
osrdyne	2.71% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	86.37% <ø> (ø)

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[Castavo]

That's already something you can achieve with pydantic models using <class>.model_validate(your_dict)

[elise-chin]

That's already something you can achieve with pydantic models using <class>.model_validate(your_dict)

Thanks! I didn't know. Since we don't need to change anything, I will close the PR.