Implement plugin mechanism for actuators and sensors.

PiperOrigin-RevId: 474874088
Change-Id: I65a8ffdf845f4fa0f8266c165883a747ab5812d8

src/engine/engine_util_misc.c

@@ -831,6 +831,9 @@ const char* mju_type2Str(int type) {
   case mjOBJ_KEY:
     return "key";
 
+  case mjOBJ_PLUGIN:
+    return "plugin";
+
   default:
     return 0;
   }
@@ -932,6 +935,10 @@ int mju_str2Type(const char* str) {
     return mjOBJ_KEY;
   }
 
+  else if (!strcmp(str, "plugin")) {
+    return mjOBJ_PLUGIN;
+  }
+
   else {
     return mjOBJ_UNKNOWN;
   }