[Extension] Implement set_objective_function instead of set_objective

[blegat]

It implement set_objective_sense so implementing set_objective was not appropriate as there is a fallback to set_objective_sense and set_objective_function.

[codecov]

Codecov Report

Merging #2247 into master will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2247      +/-   ##
==========================================
- Coverage   91.30%   91.22%   -0.08%     
==========================================
  Files          42       42              
  Lines        4186     4253      +67     
==========================================
+ Hits         3822     3880      +58     
- Misses        364      373       +9     

Impacted Files	Coverage Δ
src/objective.jl	91.66% <100.00%> (ø)
src/Containers/SparseAxisArray.jl	77.88% <0.00%> (-0.69%)	⬇️
src/Containers/DenseAxisArray.jl	87.50% <0.00%> (+3.03%)	⬆️

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