Add SDL for client directives

[jasonpaulos]

Since apollographql/apollo-tooling#1433 was merged for the Apollo VS Code extension, which by default requires definitions for all directives, I believe apollo-client should provide definitions for its custom directives. These definitions can be used as follows in the apollo.config.js file that the VS Code extension reads:

var apolloClientSchema = require.resolve('apollo-client/schema.graphql');

module.exports = {
  client: {
    name: 'Space Explorer [web]',
    service: 'apollographql-8341',
    includes: [
      'src/**/*.{ts,tsx,js,jsx,graphql,gql}',
      apolloClientSchema
    ]
  }
};

While the above pull request makes an exception for apollo-client by automatically including a version of the apollo-client directives defined here, I believe that the definitions should ideally live in this repository so that they can grow with the apollo-client codebase. Additionally, the presence of a schema.graphql file will allow users to include the directives as shown above, in case they want to override the Apollo directives provided by the extension.

To my understanding, the addition of schema.graphql will increase the apollo-client package size, but should not result in an increase to the bundle size of any app using apollo-client, since schema.graphql is used only during build time.

I'm curious to hear any feedback 🙂

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Here is a similar PR for apollo-link-rest: apollographql/apollo-link-rest#228

[hwillson]

Thanks @jasonpaulos!