Graphql-playground: Support the @defer directive in GraphQL Playground

[divyenduz]

This issue pertains to the following package(s):

• [ ] GraphQL Playground - Electron App
• [ ] GraphQL Playground HTML
• [x] GraphQL Playground
• [ ] GraphQL Playground Express Middleware
• [ ] GraphQL Playground Hapi Middleware
• [ ] GraphQL Playground Koa Middleware
• [ ] GraphQL Playground Lambda Middleware

Support for @defer directive.
https://blog.apollographql.com/introducing-defer-in-apollo-server-f6797c4e9d6e

[farzer]

also could not load the custom directives

[ssiemens-hm]

Custom directives are not shown in the schema. But via introspection the directives can be queried. Would look forward to see the directives even in the schema.