Ability to add directives on extended enum values

This is currently disallowed by the specification:

```graphql
enum Foo {
bar
baz
}

extend enum Foo {
baz @myDirective
}
```

From https://spec.graphql.org/draft/#sec-Enum-Extensions.Type-Validation:
> 3. All values of an Enum type extension must not already be a value of the original Enum.

Our immediate use-case is that we generate code on the client side and would like to allow renaming certain enum values (in case their name is a reserved keyword, or clashes with other fields in the generated code).

Has this ever come up and would it make sense to allow it?