✨ Add customGate flag and optimizer functionality to flatten only custom gates

[DRovara]

Description

This pull request adds the customGate flag to the CompoundOperation class. This flag is set for CompoundOperations that were constructed from custom gate calls.

The pull request also provides new functionality to the flattenOperations optimizer. Now, a second parameter can be used to indicate that only custom gates should be flattened. It makes sense to put this functionality here as outside of the CircuitOptimizer class, certain protected members cannot be accessed anymore.

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: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.6%. Comparing base (3ce5689) to head (6c76ad1).
Report is 144 commits behind head on main.

Files with missing lines	Patch %	Lines
src/operations/CompoundOperation.cpp	83.3%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #651     +/-   ##
=======================================
- Coverage   91.6%   91.6%   -0.1%     
=======================================
  Files        148     148             
  Lines      14755   14759      +4     
  Branches    2369    2370      +1     
=======================================
+ Hits       13522   13525      +3     
- Misses      1233    1234      +1     

Flag	Coverage Δ
cpp	91.3% <90.9%> (-0.1%)	⬇️
python	99.7% <ø> (ø)

Files with missing lines	Coverage Δ
include/mqt-core/operations/CompoundOperation.hpp	100.0% <ø> (ø)
src/CircuitOptimizer.cpp	88.9% <100.0%> (+<0.1%)	⬆️
src/parsers/QASM3Parser.cpp	90.5% <100.0%> (ø)
src/operations/CompoundOperation.cpp	85.9% <83.3%> (-0.6%)	⬇️

[burgholzer]

Many thanks for this nice addition.
I just pushed two commits with a few cleanups.
Once CI passes, this is good to go in 🚀