Error in "Loading schema for ...": GraphQLSchemaValidationError: Unknown directive "@aws_subscribe".

Hi,
I have a problem while adding the extension with the local schema file.
We are using aws-app-sync. Below is the example schema

```
Type Article{
title:String!
description:String
author:Author
}

type Subscription {
updateTitleSubscription(domain: String!): Article
@aws_subscribe(mutations: ["updateTitle"])
}
```
my `apollo.config.js`
```
module.exports = {
client: {
includes: [
'./src/**/stores/**/*.js',
'./src/**/stores/**/*.ts',
'./src/**/stores/**/*.tsx'
],
service: {
localSchemaFile: './src/graphql/schema.graphql'
}
}
}

```
Is there a way to add custom decorators like above or shall i remove them to work?
TIA :smiley:

┆Issue is synchronized with this [Jira Task](https://apollographql.atlassian.net/browse/NEBULA-938) by [Unito](https://www.unito.io)

Start with apollo.config.js and src/graphql/schema.graphql, then reproduce local-schema loading for the @aws_subscribe directive. Trace how the extension validates this schema; done means the schema loads without the unknown-directive error or the supported handling for the directive is documented.

Written by the indexing model from the issue text.