On Linux, thread IDs were not properly assigned with the current approach.
The line:
`std::thread new_thread(&Thread::callback, _thread_id_hash(thread.get_id()), p_settings, p_callback, p_user);`
does not work because the thread ID is not assigned until the thread starts.
This PR changes the behavior to use manually generated thread IDs. Additionally, if a thread is (or may have been created) outside Godot, the method `Thread::attach_external_thread` was added.
a37c30dfc9