feat(util): Add overloaded helper (#3816)

This is useful in combination with `std::visit`. With this, you can do something like 

```cpp
  struct A {};
  std::variant<int, double, A> var;

  var = 42;

  std::visit(overloaded{
                 [](int) { BOOST_CHECK(true); },
                 [](double) { BOOST_CHECK(false); },
                 [](A) { BOOST_CHECK(false); },
             },
             var);
```

which can be convenient.

I'm using this in #3869

Core/include/Acts/Utilities/Helpers.hpp

@@ -204,4 +204,21 @@ bool rangeContainsValue(const R& range, const T& value) {
   return std::ranges::find(range, value) != std::ranges::end(range);
 }
 
+/// Helper struct that can turn a set of lambdas into a single entity with
+/// overloaded call operator. This can be useful for example in a std::visit
+/// call.
+/// ```cpp
+/// std::visit(overloaded{
+///  [](const int& i) { std::cout << "int: " << i << std::endl; },
+///  [](const std::string& s) { std::cout << "string: " << s << std::endl; },
+/// }, variant);
+/// ```
+template <class... Ts>
+struct overloaded : Ts... {
+  using Ts::operator()...;
+};
+
+template <class... Ts>
+overloaded(Ts...) -> overloaded<Ts...>;
+
 }  // namespace Acts

Tests/UnitTests/Core/Utilities/HelpersTests.cpp

@@ -27,6 +27,7 @@
 #include <string>
 #include <tuple>
 #include <utility>
+#include <variant>
 #include <vector>
 
 using namespace Acts::VectorHelpers;
@@ -310,6 +311,20 @@ BOOST_AUTO_TEST_CASE(incidentAnglesTest) {
   }
 }
 
+BOOST_AUTO_TEST_CASE(Overloaded) {
+  struct A {};
+  std::variant<int, double, A> var;
+
+  var = 42;
+
+  std::visit(overloaded{
+                 [](int) { BOOST_CHECK(true); },
+                 [](double) { BOOST_CHECK(false); },
+                 [](A) { BOOST_CHECK(false); },
+             },
+             var);
+}
+
 BOOST_AUTO_TEST_SUITE_END()
 
 }  // namespace Acts::Test