Support subscriptions in SchemaLink

[bennypowers]

This issue on the old Apollo Link repo sums it up: there are cases (like testing, and demos) in which a SchemaLink user wants to add a subscription to their executable schema:

import { SchemaLink } from '@apollo/client/link/schema';
import { makeExecutableSchema } from '@graphql-tools/schema';
import { addMocksToSchema, createMockStore } from '@graphql-tools/mock';
import { ApolloClient, InMemoryCache } from '@apollo/client/core';
import { EventIterator } from 'event-iterator';

export interface User {
  id: number;
  name: string;
  status?: 'DELETED';
};

const typeDefs = `
  type User {
    name: String
    id: ID
  }

  type Subscription {
    userAdded: User
  }

  schema {
    subscription: Subscription
  }
`;

// mock DB
const USERS: User[] = [
  { id: 1, name: 'Neil' }
];

const getNextUserId = () => Math.max(...USERS.map(x => x.id)) + 1;

function getIterator() {
  return new EventIterator(
    ({ push }) => {
      document.querySelector('button').addEventListener('click', push);
      return () => document.querySelector('button').removeEventListener('click', push);
    }
  );
}

const schema = makeExecutableSchema({
  typeDefs,
  resolvers: {
    Subscription: {
      userAdded: {
        subscribe: () => getIterator(),
        resolve: () => makeNextUser()
      }
    }
  }
});


const store = createMockStore({ schema });

function makeNextUser() {
  const id = getNextUserId();
  return {
    name: store.get('User', id, 'name'),
    id: store.get('User', id, 'id'),
  };
}

export const client = new ApolloClient({
  cache: new InMemoryCache(),
  link: new SchemaLink({
    schema: addMocksToSchema({
      preserveResolvers: true,
      store,
      schema,
    }),
  }),
});

window.__APOLLO_CLIENT__ = client;

The above code looks like it should work, but it appears that the subscribe callback under resolvers.Subscription.userAdded never gets called at all.

The above-linked issue also links to a (now presumably out-of-date) Pull Request. Could that PR be reviewed/modified to bring subscriptions into SchemaLink?