Let extensions intercept schemas

[kamilkisiela]

export const MyExtension: GraphQLExtensionDeclaration = api => {
  api.loaders.schema.use(document => addDirective(document));
 
  return {
    name: 'my-extension',
  };
};

[theguild-bot]

The latest changes of this PR are available as alpha in npm: 0.0.0-experimental-5ea0951.511

Quickly update your package.json by running:

npx match-version graphql-config 0.0.0-experimental-5ea0951.511

[zth]

This is great, thank you!