⚡🚨 make qiskit_optimization import lazy

[burgholzer]

This PR slightly adjusts the imports in the utils.py file so that qiskit_optimization is important lazily.
Besides slightly improving the general import speed, this also avoids a docplex syntax warning appearing on Python 3.12+ when importing mqt.bench.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.1%. Comparing base (d24e61d) to head (3442b82).
Report is 65 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #383   +/-   ##
=====================================
  Coverage   93.0%   93.1%           
=====================================
  Files         48      48           
  Lines       2438    2438           
=====================================
+ Hits        2269    2270    +1     
+ Misses       169     168    -1     

Flag	Coverage Δ
python	93.1% <100.0%> (+<0.1%)	⬆️

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