Add mj_extractState as a public API function.

This function allows the caller to extract a subset of components of a state previously obtained via `mj_getState` without having to first write it back into `mjData`.

PiperOrigin-RevId: 822165077
Change-Id: I6433261f4f5ec2e8d024bb7dee8f7dbf1e62666c

src/engine/engine_support.h

@@ -41,6 +41,10 @@ MJAPI int mj_stateSize(const mjModel* m, unsigned int sig);
 // get state
 MJAPI void mj_getState(const mjModel* m, const mjData* d, mjtNum* state, unsigned int sig);
 
+// extract a sub-state from a state
+MJAPI void mj_extractState(const mjModel* m, const mjtNum* src, unsigned int srcsig,
+                           mjtNum* dst, unsigned int dstsig);
+
 // set state
 MJAPI void mj_setState(const mjModel* m, mjData* d, const mjtNum* state, unsigned int sig);