refactor!: std::unique_ptr to std::optional in Acts::PropagatorResult

[benjaminhuth]

This changes the type that stores the parameters and covariance matrix in the Acts::PropagatorResult.

I'm not sure why we have std::unique_ptr here, maybe this is from pre-C++17 where std::optional was not available? However, both from usibility (not copy-able) and from performance perspective (heap allocation) this does not make sense to me, but maybe I forget something...

[andiwand]

I guess the only advantage of std::unique_ptr is that the struct is more densely packed. but that doesn't look like a critical property here

[codecov]

Codecov Report

Merging #1622 (3d18c3f) into main (5c07814) will decrease coverage by 0.04%.
The diff coverage is 18.75%.

❗ Current head 3d18c3f differs from pull request most recent head dfdd16d. Consider uploading reports for the commit dfdd16d to get more accurate results

@@            Coverage Diff             @@
##             main    #1622      +/-   ##
==========================================
- Coverage   48.65%   48.60%   -0.05%     
==========================================
  Files         381      381              
  Lines       20780    20826      +46     
  Branches     9518     9553      +35     
==========================================
+ Hits        10110    10122      +12     
- Misses       4099     4101       +2     
- Partials     6571     6603      +32     

Impacted Files	Coverage Δ
Core/include/Acts/Propagator/Propagator.hpp	90.24% <ø> (ø)
Core/include/Acts/Propagator/Propagator.ipp	47.22% <0.00%> (-5.28%)	⬇️
...re/include/Acts/Vertexing/ImpactPointEstimator.hpp	100.00% <ø> (ø)
...re/include/Acts/Vertexing/ImpactPointEstimator.ipp	35.67% <0.00%> (-0.59%)	⬇️
.../include/Acts/Vertexing/HelicalTrackLinearizer.ipp	26.80% <14.28%> (-0.75%)	⬇️
...clude/Acts/Vertexing/AdaptiveMultiVertexFitter.ipp	46.00% <33.33%> (ø)
...re/include/Acts/TrackFitting/GaussianSumFitter.hpp	64.63% <100.00%> (ø)
Core/src/TrackFitting/GsfUtils.cpp	27.27% <0.00%> (-17.18%)	⬇️
Core/src/TrackFinding/MeasurementSelector.cpp	21.42% <0.00%> (-11.91%)	⬇️
Core/src/TrackFitting/GainMatrixUpdater.cpp	15.15% <0.00%> (-4.85%)	⬇️
... and 11 more

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[benjaminhuth]

@andiwand After fixing the unittests etc, could you reapprove
@paulgessinger This should be good to go in now, so would be cool to wait with the next major release until this is in.

[github-actions]

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