Add user sensor bindings.

Stop force-setting nusersensor, instead allow MuJoCo to infer the size.

PiperOrigin-RevId: 494783883
Change-Id: Ia2a1b1803b2904f2dded7b790d01eb6f1920dbaf

unity/Runtime/Importer/MjcfImporter.cs

@@ -376,6 +376,9 @@ public class MjcfImporter {
             break;
         }
         break;
+      case "user":
+        sensorType = typeof(MjUserSensor);
+        break;
       default:
         Debug.Log($"The importer does not yet support sensor <{node.Name}>.");
         break;