chore: specialize Atomic<std::shared_ptr<T>>

[Nerixyz]

Since #5128 refactors message parsing, it would be nice to do that off the main thread. To do this safely, we can utilize the Atomic class to provide shared state for workers (similar to the MessageLayoutContext).

C++ 20 added std::atomic<std::shared_ptr<T>>. Not every standard library implementation has added this yet, so it's behind an #ifdef. To ensure sane use, Atomic<std::shared_ptr<T>> requires T to be const (otherwise the atomic is kinda useless). Since only the MSVC STL and GCC libstdc++ implement this and ship in compiler versions that have concepts, it's fine to require these too.

Neither implementation is lock-free, but both are better than using a std::mutex (e.g. the implementation in the STL spins on a single bit compared to a mutex which uses a shared read-write lock).