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

test/fixture.cc

@@ -51,6 +51,7 @@ using ::testing::Truly;
 bool IsBenignWarning(const std::string& msg) {
   static const char* const kBenignWarnings[] = {
       "is not rigid and has no equality constraints",
+      "Attach conflict",
   };
   for (const char* warning : kBenignWarnings) {
     if (absl::StrContains(msg, warning)) {