✨ switch to mqt-core Python package

[burgholzer]

Description

This PR switches the project to use the newly developed mqt-core Python package.
As part of this change, it switches from purely using the git submodule to using FetchContent (as advised in cda-tum/mqt-qcec#302), which also allows to use the libraries directly provided by the mqt-core Python package.
This means that any Python package build will only require the mqt-core package as a build and runtime dependency and won't have to rely on the git submodule (or a fetched version of mqt-core) any more.
Any regular C++ build will use the FetchContent CMake module.

The change brings a range of advantages:

• Uses the new load function of mqt-core that allows to load any supported circuit format as input and has better handling for Qiskit QuantumCircuit objects.
• Simplifies the interfaces of the DDSIM Python bindings
• Allows OpenQASM3 as an input format.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (a50b525) 92.7% compared to head (60cd68f) 92.9%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #336     +/-   ##
=======================================
+ Coverage   92.7%   92.9%   +0.1%     
=======================================
  Files         35      35             
  Lines       2660    2666      +6     
  Branches     345     345             
=======================================
+ Hits        2468    2477      +9     
+ Misses       192     189      -3     

Flag	Coverage Δ
cpp	94.6% <ø> (-0.1%)	⬇️
python	88.2% <84.2%> (+0.5%)	⬆️

Files	Coverage Δ
src/mqt/ddsim/__init__.py	100.0% <100.0%> (ø)
src/mqt/ddsim/hybridqasmsimulator.py	91.2% <100.0%> (+0.3%)	⬆️
src/mqt/ddsim/primitives/estimator.py	95.0% <100.0%> (+<0.1%)	⬆️
src/mqt/ddsim/qasmsimulator.py	98.9% <100.0%> (+<0.1%)	⬆️
src/mqt/ddsim/unitarysimulator.py	87.8% <100.0%> (+0.3%)	⬆️
src/mqt/ddsim/pathqasmsimulator.py	52.7% <57.1%> (+1.8%)	⬆️

... and 1 file with indirect coverage changes