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

introspect/enums.py

@@ -642,6 +642,16 @@ ENUMS: Mapping[str, EnumDecl] = dict([
              ('mjFONT_BIG', 2),
          ]),
      )),
+    ('mjtTaskStatus',
+     EnumDecl(
+         name='mjtTaskStatus',
+         declname='enum mjtTaskStatus_',
+         values=dict([
+             ('mjTASK_NEW', 0),
+             ('mjTASK_QUEUED', 1),
+             ('mjTASK_COMPLETED', 2),
+         ]),
+     )),
     ('mjtButton',
      EnumDecl(
          name='mjtButton',