Refactor thread pool implementation.

- Make mjTask non-opaque and remove C++ Task class.
- Make C++ thread pool a subclass of a skeletal mjThreadPool C struct.
- Make the mjTask status enum more consistent with the rest of MuJoCo.
- Change mju_threadPoolEnqueue to take just the mjTask. Users must now prepare the mjTask by assigning the function pointer and argument into the struct.
- Rename files in thread/ to be more consistent with the rest of MuJoCo.
- Run threading tests in CMake.
- Allow use of C++20 designated initializers.

Otherwise the functionality remains identical.

PiperOrigin-RevId: 564374675
Change-Id: I37c9894566bc39faf217e5aa97a4e2713a70e467

cmake/MujocoOptions.cmake

@@ -99,9 +99,12 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR (CMAKE_CXX_COMPILER_ID MATCHES "Clang
   if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     # Set -Wimplicit-fallthrough=5 to only allow fallthrough annotation via __attribute__.
     set(EXTRA_COMPILE_OPTIONS ${EXTRA_COMPILE_OPTIONS} -Wimplicit-fallthrough=5
-                              -Wno-maybe-uninitialized
+                              -Wno-maybe-uninitialized -Wno-c++20-extensions
     )
   endif()
+  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    set(EXTRA_COMPILE_OPTIONS ${EXTRA_COMPILE_OPTIONS} -Wno-c++20-designator)
+  endif()
 endif()
 
 include(MujocoHarden)