Can not define an interface with a field that is then specialised as an enum field.

In previous versions of relay it was possible to define interfaces that defined a `String!` field, and that field could then be specialized to an enum object the implemented this interface, such that the compiler would treat the specialized value as a sub-type of the `String!`. For example, I could define this interface:

```
interface StatusChangeable {
status: String!
allowedActions: [String!]!
}
```

And then use it as follows:

```
enum UserStatus {
active
banned
inactive
}

type User implements StatusChangeable {
status: UserStatus!
}
```

This way any code that used the `User` type directly could be typed properly to handle the specific values that `User` supports, while code that cared only about the `StatusChangeable` didn't have to concern itself with the specialized values.

It would be great if I could define the `interface StatusChangeable` with either a generic `String!` status, or even more ideally, with an enum that is a super-set of the specialized enum, such as:

```
enum AllStatuses {
active
banned
inactive
other
statuses
here
}
```

Otherwise the only option is to update all the types that implement `StatusChangeable` to define `status` as a `String!` which loses all the specificity and type-safety, or to avoid using an interface which can create a lot of duplicate code that could easily be prevented with an single interface.

If [is_named_type_subtype_of](https://github.com/facebook/relay/blob/1e60e4716a5e6dccbba4c1c23a83a296b3b28542/compiler/crates/schema/src/graphql_schema.rs#L160) handled the Enum subset scenario, this could be done fairly easily:

```
(Type::Enum(sub_id), Type::Enum(super_id)) => {
let sub_enum = self.enum_(sub_id);
let super_enum = self.enum_(super_id);
// Ensure all values in the sub enum are present in the super enum
sub_enum.values.iter().all(|sub_value| {
super_enum.values.iter().any(|super_value| super_value.value == sub_value.value)
})
}
```

If this approach is acceptable I can submit a PR.