Allow plugin actuators to have activation state.

Accept new attributes on the plugin/actuator tag:
  - dyntype
  - dynprm
  - actlimited
  - actrange
  - actearly

This addresses some of the requests in #1098.

PiperOrigin-RevId: 584866281
Change-Id: I4f7559e446888216093efba4ced9119cfc0528a4

include/mujoco/mjplugin.h

@@ -107,6 +107,15 @@ struct mjpPlugin_ {
   // called by mjv_updateScene (optional)
   void (*visualize)(const mjModel*m, mjData* d, const mjvOption* opt, mjvScene* scn, int instance);
 
+  // methods specific to actuators (optional)
+
+  // dimension of the actuator state for the plugin (excluding state from actuator's dyntype)
+  int (*actuator_actdim)(const mjModel*m, int instance, int actuator_id);
+
+  // updates the actuator plugin's entries in act_dot
+  // called after native act_dot is computed and before the compute callback
+  void (*actuator_act_dot)(const mjModel* m, mjData* d, int instance);
+
   // methods specific to signed distance fields (optional)
 
   // signed distance from the surface