Federation: Middlewares have no effect?

I am trying to use `graphql-shield` on my gateway server. It seems like while the shield is called, the error from the shield does not make it to the user. My guess is that the `executor` is somehow overriding the result from the middleware.

Is there a documented way of using middlewares with apollo-gateway?

If it is currently not possible to use middlewares with apollo-gateway, this is a feature request :grin:

```typescript
const { schema: federatedSchema, executor } = await gateway.load();

const server = new ApolloServer({
schema: applyMiddleware(
federatedSchema,
shield(
{
Query: deny,
Mutation: deny
},
{
debug: true
}
)
),
executor
});

```

Start with the gateway.load() result and its executor, then trace how the ApolloServer setup combines that executor with applyMiddleware and shield. Reproduce the shown TypeScript configuration and determine whether middleware errors reach the user; done means either middleware behavior is supported and documented or the limitation and required feature are clearly defined.

Written by the indexing model from the issue text.