Add policies for global attribute conflict resolution upon attach

For example when loading parent_merge.xml:

```
WARNING: Attach conflict when attaching 'child' to 'parent_merge', policy is 'merge'
timestep: parent has 0.005, child has 0.002, taking the minimum
iterations: parent has 50, child has 100, taking the maximum
flag 'Damper': added from child
```

When loading parent_error.xml:

```
XML Error: Attach conflict when attaching 'child' to 'parent_error', policy is 'error'
timestep: parent has 0.005, child has 0.002
iterations: parent has 50, child has 100
Element 'attach', line 10
```
PiperOrigin-RevId: 933620810
Change-Id: Ib477863b5ef763474d27fb4be5a4148be1d5d500

unity/Runtime/Bindings/MjBindings.cs

@@ -583,6 +583,11 @@ public enum mjtOrientation : int{
   mjORIENTATION_ZAXIS = 3,
   mjORIENTATION_EULER = 4,
 }
+public enum mjtConflict : int{
+  mjCONFLICT_WARNING = 0,
+  mjCONFLICT_MERGE = 1,
+  mjCONFLICT_ERROR = 2,
+}
 public enum mjtCTimer : int{
   mjCTIMER_TOTAL = 0,
   mjCTIMER_ASSETS = 1,
@@ -5865,6 +5870,7 @@ public unsafe struct mjsCompiler_ {
   public fixed int inertiagrouprange[2];
   public byte saveinertial;
   public int alignfree;
+  public int conflict;
   public mjLROpt_ LRopt;
   public void* meshdir;
   public void* texturedir;