Use OnePlugin for parsing Composite and Flexcomp plugins.

PiperOrigin-RevId: 608265172
Change-Id: I053441a9e534d81c4847ad60fe4a3f7dce87350c

src/user/user_api.h

@@ -761,7 +761,7 @@ MJAPI mjmTuple* mjm_addTuple(mjmModel* model);
 MJAPI mjmKey* mjm_addKey(mjmModel* model);
 
 // Add plugin to model.
-MJAPI mjElement mjm_addPlugin(mjmModel* model);
+MJAPI mjmPlugin* mjm_addPlugin(mjmModel* model);
 
 // Add default to model.
 MJAPI mjmDefault* mjm_addDefault(mjmModel* model, const char* classname, int parentid);
@@ -895,6 +895,9 @@ MJAPI void mjm_defaultTuple(mjmTuple& tuple);
 // Default keyframe attributes.
 MJAPI void mjm_defaultKey(mjmKey& key);
 
+// Default plugin attributes.
+MJAPI void mjm_defaultPlugin(mjmPlugin& plugin);
+
 #ifdef __cplusplus
 }
 #endif