⬆️ update mqt-core

[burgholzer]

Description

This PR updates the mqt-core submodule and brings in some further advancements from #355.
Specifically, it switches to using FetchContent for obtaining mqt-core.
Per default, it is pointed to the vendored submodule via an overridable CMake variable.

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

All modified and coverable lines are covered by tests ✅

Comparison is base (5455bc5) 92.6% compared to head (e5de0e1) 92.7%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #337     +/-   ##
=======================================
+ Coverage   92.6%   92.7%   +0.1%     
=======================================
  Files         35      35             
  Lines       2683    2666     -17     
  Branches     347     346      -1     
=======================================
- Hits        2486    2473     -13     
+ Misses       197     193      -4     

Flag	Coverage Δ
cpp	94.6% <ø> (+0.1%)	⬆️
python	87.7% <ø> (-0.3%)	⬇️

Files	Coverage Δ
src/mqt/ddsim/hybridqasmsimulator.py	90.9% <ø> (-0.5%)	⬇️
src/mqt/ddsim/pathqasmsimulator.py	50.9% <ø> (-1.4%)	⬇️
src/mqt/ddsim/qasmsimulator.py	98.9% <ø> (-0.1%)	⬇️
src/mqt/ddsim/unitarysimulator.py	87.5% <ø> (-0.6%)	⬇️

... and 2 files with indirect coverage changes