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

test/xml/xml_native_reader_test.cc

@@ -375,7 +375,7 @@ TEST_F(UserDataTest, RequiresControlRange) {
   std::array<char, 1024> error;
   mjModel* model = LoadModelFromString(xml, error.data(), error.size());
   ASSERT_THAT(model, IsNull());
-  EXPECT_THAT(error.data(), HasSubstr("invalid control range for actuator"));
+  EXPECT_THAT(error.data(), HasSubstr("required attribute missing: 'ctrlrange'"));
 }
 
 TEST_F(UserDataTest, PositiveControlRange) {