Threading hooks are not exposed

In the Thread class, Godot sets up more things than just running the work function:
https://github.com/godotengine/godot/blob/47ef0549ee490bca066ac00587076f123d973a55/core/os/thread.cpp#L53
Notably, it setups script debugging.

This isn't a problem if an extension uses the `Thread` class, or if you dont plan to run anything Godot-related in that thread.
However it becomes a problem if anything else needs to be used. [WorkedThreadPool](https://github.com/godotengine/godot-cpp/blob/master/include/godot_cpp/templates/thread_work_pool.hpp) for example is pure C++ and does not use Godot objects, therefore it uses `std::thread`, but lacks the hooks Godot is setting up (while the [core implementation](https://github.com/godotengine/godot/blob/master/core/object/worker_thread_pool.h) uses `Thread`). This could lead to unforeseen issues related to the absence of this setup.
If threads come from a third party library, there is also no way to set this up, unless said library allows to re-implement its threads using the `Thread` class. The latter is also a bit inconvenient as it requires the function to run to be a method from a registered Godot object.

Perhaps GDExtension could expose two functions in order to tell Godot when a custom thread starts and ends, so that it can be made aware of them if needed?