Use field identifiers when code gen struct items

Field identifiers were not being used. Instead, the order of field
identifiers was used, with ordinal (0 to N-1) indexes.

This is likely true of other code generated elements, but this PR only
fixes it for struct items.

This PR requires PR project-chip#326 in the third_party/zap/repo to function,
as that PR actually includes the field identifier in the query
results.

Tested by running Matter cert tests, and also by inspecting the
generated code for correctness.

src/app/zap-templates/templates/app/cluster-objects.zapt

@@ -54,7 +54,7 @@ namespace Structs {
 namespace {{asUpperCamelCase name}} {
     enum class Fields {
     {{#zcl_struct_items}}
-    k{{asUpperCamelCase label}} = {{index}},
+    k{{asUpperCamelCase label}} = {{fieldIdentifier}},
     {{/zcl_struct_items}}
     };