Add validation rule for disabling introspection

[danielrearden]

It would be helpful if the library included a validation rule that would throw when an introspection field is requested. Something like what Apollo Server already uses:

const NoIntrospection = (context: ValidationContext) => ({
  Field(node: FieldDefinitionNode) {
    if (node.name.value === '__schema' || node.name.value === '__type') {
      context.reportError(
        new GraphQLError(
          'GraphQL introspection is not allowed by Apollo Server, but the query contained __schema or __type. To enable introspection, pass introspection: true to ApolloServer in production',
          [node],
        ),
      );
    }
  },
});

The validation rule could then be optionally added when using libraries like express-graphql to effectively disable introspection.