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

src/xml/xml_base.h

@@ -48,6 +48,7 @@ extern const int stage_sz;
 extern const int datatype_sz;
 extern const int camout_sz;
 extern const int reduce_sz;
+extern const int conflict_sz;
 extern const mjMap angle_map[];
 extern const mjMap enable_map[];
 extern const mjMap bool_map[];
@@ -86,7 +87,7 @@ extern const mjMap meshtype_map[];
 extern const mjMap meshinertia_map[];
 extern const mjMap flexself_map[];
 extern const mjMap elastic2d_map[];
-
+extern const mjMap conflict_map[];
 
 //---------------------------------- Base XML class ------------------------------------------------
 

src/xml/xml_native_writer.cc

@@ -1034,6 +1034,8 @@ void mjXWriter::Compiler(XMLElement* root) {
   if (!model->compiler.autolimits) {
     WriteAttrTxt(section, "autolimits", "false");
   }
+  WriteAttrKey(section, "conflict", conflict_map, conflict_sz,
+               model->compiler.conflict, mjCONFLICT_WARNING);
 }