Extend schema

[Yosuke Kurami]

We need ways to extend server-side schema. For example, with apollo local state , developers write operations such as:

const query = gql`
  query LocalQuery {
     userForm @client {
       name
       age
     }
  }
`;

But for now this plugin reports errors that @client and userForm type are not defined in server-side schema.

To suppress this ( or to complete local type fields or directives ), this plugin needs to support to extend server-side schema with client-only schema files.

## local.graphql

directive @client on FIELD
type UserForm {
  name: String!
  age: Int!
}

extend type Query {
  userForm: UserForm!
}

/* tsconfig.json */

"plugins": [
  { 
     "name": "ts-graphql-plugin",
     "schema": "./schema.graphql", /* derived from server */
     "localSchema": ["./local.graphql"], /* client only schema */
  } 
]