feat: Add polymorphic value like container

[paulgessinger]

This PR introduces a type PolymorphicValue<T> that behaves very similar to std::unique_ptr<T>, but preserves knowlegde of how to copy the value. In fact, PolymorphicValue only works with copyable types.

This allows more convenient handling of interfaces, for types that are copyable and where copying does not have to be avoided.
It looks like this:

struct Base {};
struct Derived {};

std::unique_ptr<Base> up1 = std::make_unique<Derived>();
std::unique_ptr<Base> up2 = up1; // this does not work

Acts::PolymorphicValue<Base> pv1 = Acts::makePolymorphicValue<Derived>();
Acts::PolymorphicValue<Base> pv2 = pv1; // this makes a copy of the stored object

The reason why I'm adding this is that pybind11 does not support std::unique_ptr as arguments/values on the C++ side, since python lacks a mechanism to release ownership, which would be required in this case. There's a limited number of cases where this is a problem, and where we don't actually care if we copy the stored value.

WIP until I finalize this.

[codecov]

Codecov Report

Merging #855 (da904dd) into main (c7ad7e8) will increase coverage by 0.12%.
The diff coverage is 68.62%.

@@            Coverage Diff             @@
##             main     #855      +/-   ##
==========================================
+ Coverage   48.53%   48.65%   +0.12%     
==========================================
  Files         329      330       +1     
  Lines       16974    17076     +102     
  Branches     8025     8053      +28     
==========================================
+ Hits         8239     8309      +70     
- Misses       3083     3089       +6     
- Partials     5652     5678      +26     

Impacted Files	Coverage Δ
Core/include/Acts/Utilities/PolymorphicValue.hpp	68.62% <68.62%> (ø)

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[paulgessinger]

@HadrienG2 would you be available to review?

[HadrienG2]

Makes sense.

[gagnonlg]

@paulgessinger how come the CI didn't try a build for this PR? It broke the build due to a missing #include <optional> in PolymorphicValueTest.cpp.