Add adhesion actuators.

- Adhesion actuators using contact normals as force transmission mechanism.
- Related video: https://youtu.be/HdBue4MUZys

Closes #229

PiperOrigin-RevId: 464389367
Change-Id: I9f69b3cd152d957e8f65870d208788463c036a6d

src/xml/xml_native_writer.cc

@@ -554,6 +554,10 @@ void mjXWriter::OneActuator(XMLElement* elem, mjCActuator* pact, mjCDef* def) {
       WriteAttrTxt(elem, "site", pact->target);
       break;
 
+    case mjTRN_BODY:
+      WriteAttrTxt(elem, "body", pact->target);
+      break;
+
     default:        // SHOULD NOT OCCUR
       break;
     }