Add <damper> actuator shortcut and related mjGAIN_AFFINE.

**Rationale**

A general `mjGAIN_AFFINE` actuator adds the term `force = (c + kp * length + kv * velocity) * ctrl`. The damper shortcut restricts it to `kv * velocity * ctrl` and sets the `ctrllimited` attribute to true.

**API**

```xml
<actuator>
  <damper ctrlrange="0 1" kv="1"/>
</actuator>
```

* Required attributes: `ctrlrange` (>=0)
* Optional attributes: `kv` (>=0)

PiperOrigin-RevId: 455366866
Change-Id: I88773abbeccabb6442f215c9fc05e205df63f692

unity/Runtime/Bindings/MujocoBindings.cs

@@ -227,6 +227,7 @@ public enum mjtDyn : int{
 }
 public enum mjtGain : int{
   mjGAIN_FIXED = 0,
+  mjGAIN_AFFINE = 1,
   mjGAIN_MUSCLE = 1,
   mjGAIN_USER = 2,
 }

unity/Tests/Editor/Components/MjActuatorTests.cs

@@ -165,6 +165,7 @@ public class MjGeneralActuatorTests {
   }
 
   [TestCase(MujocoLib.mjtGain.mjGAIN_FIXED, "fixed")]
+  [TestCase(MujocoLib.mjtGain.mjGAIN_AFFINE, "affine")]
   [TestCase(MujocoLib.mjtGain.mjGAIN_MUSCLE, "muscle")]
   [TestCase(MujocoLib.mjtGain.mjGAIN_USER, "user")]
   public void GainTypeMjcf(MujocoLib.mjtGain value, string expected) {