this is allow custom allocator for obj-api structs/tables. (#4520)

added "native_custom_alloc" attribute to tables/structs, eg.

table parent_table( native_custom_alloc:"custom_alloc_name" ) {
...
}

with a custom allocator defined as

template <typename T> class custom_alloc_name : public std::allocator<T> {
public:

 typedef T*       pointer;

 template <class U>
 struct rebind {
  typedef custom_alloc_name<U> other;
 };

 pointer allocate(const std::size_t n) {
      return ....;
 }

 void deallocate(T* ptr, std::size_t n) {
    ...
 }

 custom_alloc_name() throw() {}
 template <class U> custom_alloc_name(const custom_alloc_name<U>&) throw() {}
};
};

src/idl_parser.cpp

@@ -676,6 +676,10 @@ CheckedError Parser::ParseField(StructDef &struct_def) {
     }
   }
 
+  auto field_native_custom_alloc = field->attributes.Lookup("native_custom_alloc");
+  if (field_native_custom_alloc)
+    return Error("native_custom_alloc can only be used with a table or struct definition");
+
   field->native_inline = field->attributes.Lookup("native_inline") != nullptr;
   if (field->native_inline && !IsStruct(field->value.type))
     return Error("native_inline can only be defined on structs'");