Implement threading for island constraint solving.

Humanoids22 No threads:
 Benchmark Execution Time: 9.475905812s

Humanoids22 with 10 Threads:
 Benchmark Execution Time: 4.871352s

PiperOrigin-RevId: 571214307
Change-Id: I1f4f2c761b4ae6bc8fac1f28c6c695f6c499f339

include/mujoco/mujoco.h

@@ -1310,6 +1310,9 @@ MJAPI const mjpResourceProvider* mjp_getResourceProviderAtSlot(int slot);
 // Create a thread pool with the specified number of threads running.
 MJAPI mjThreadPool* mju_threadPoolCreate(size_t number_of_threads);
 
+// Adds a thread pool to mjData and configures it for multi-threaded use.
+MJAPI void mju_bindThreadPool(mjData* d, void* thread_pool);
+
 // Enqueue a task in a thread pool.
 MJAPI void mju_threadPoolEnqueue(mjThreadPool* thread_pool, mjTask* task);