Add plugins list to mjSpec Python API.

Fixes #2061.

PiperOrigin-RevId: 677762985
Change-Id: I8e407373e7b505d9b0f1c023e97c5ea92169bbf6

include/mujoco/mjspec.h

@@ -181,7 +181,7 @@ typedef struct mjsOrientation_ {   // alternative orientation specifiers
 
 
 typedef struct mjsPlugin_ {        // plugin specification
-  mjsElement* instance;            // element type
+  mjsElement* element;             // element type
   mjString* name;                  // name
   mjString* instance_name;         // instance name
   int plugin_slot;                 // global registered slot number of the plugin

include/mujoco/mujoco.h

@@ -1628,6 +1628,9 @@ MJAPI mjsTexture* mjs_asTexture(mjsElement* element);
 // Safely cast an element as mjsMaterial, or return NULL if the element is not an mjsMaterial.
 MJAPI mjsMaterial* mjs_asMaterial(mjsElement* element);
 
+// Safely cast an element as mjsPlugin, or return NULL if the element is not an mjsPlugin.
+MJAPI mjsPlugin* mjs_asPlugin(mjsElement* element);
+
 
 //---------------------------------- Attribute setters ---------------------------------------------