Remove mjCAlternative and user_objects dependency from xml_urdf.

PiperOrigin-RevId: 618832482
Change-Id: I626bb956cd3eb35317811b9b577cc7db128262ea

src/user/user_api.h

@@ -915,6 +915,10 @@ MJAPI void mjs_setDefault(mjElement element, mjmDefault* def);
 // Set frame.
 MJAPI void mjs_setFrame(mjElement dest, mjsFrame* frame);
 
+// Resolve alternative orientations to quat.
+MJAPI const char* mjs_resolveOrientation(double quat[4], mjtByte degree, const char* sequence,
+                                         const mjsOrientation* orientation);
+
 // Compute quat and inertia from body->fullinertia.
 MJAPI const char* mjs_setFullInertia(mjsBody* body, double quat[4], double inertia[3]);
 
@@ -924,6 +928,9 @@ MJAPI const char* mjs_setFullInertia(mjsBody* body, double quat[4], double inert
 // Default model attributes.
 MJAPI void mjs_defaultSpec(mjSpec& model);
 
+// Default orientation attributes.
+MJAPI void mjs_defaultOrientation(mjsOrientation& orient);
+
 // Default body attributes.
 MJAPI void mjs_defaultBody(mjsBody& body);