Add OnePlugin read/write functions and remove redundant plugin attributes from mjCActuator.

PiperOrigin-RevId: 525708657
Change-Id: I762de238f989b13877d5f9198c83c16ea9dc1d8b

src/xml/xml_native_writer.h

@@ -61,6 +61,7 @@ class mjXWriter : public mjXBase {
   void OneEquality(tinyxml2::XMLElement* elem,  mjCEquality* pequality, mjCDef* def);
   void OneTendon(tinyxml2::XMLElement* elem,    mjCTendon* ptendon,     mjCDef* def);
   void OneActuator(tinyxml2::XMLElement* elem,  mjCActuator* pactuator, mjCDef* def);
+  void OnePlugin(tinyxml2::XMLElement* elem,    mjCBase* object);
 
   bool writingdefaults;                       // true during defaults write
 };