Using `pg-pubsub` in schema-only mode
### Summary

I'm using PostGraphile in [schema-only mode](https://www.graphile.org/postgraphile/usage-schema/). However [custom subscriptions](https://www.graphile.org/postgraphile/subscriptions/#custom-subscriptions) don't seem to work, for two reasons:

* `createPostGraphileSchema` takes options of type `PostGraphileCoreOptions`. This type doesn't have a `pluginHook` field for adding the `@graphile/pg-pubsub` plugin (this field is present on `PostGraphileOptions`, which extends `PostGraphileCoreOptions`).

* If I ignore that and set `pluginHook` anyway, the `graphql` module throws when subscribing, with the error message `Subscription field must return Async Iterable. Received: undefined.`

It's possible to work around this issue by creating a PostGraphile HTTP request handler, and then getting the schema from there:

```
- const schema = await createPostGraphileSchema(...)
+ const schema = await postgraphile(...).getGraphQLSchema()
```

But that seems a bit convoluted, so I'm curious if I'm missing something here or if that's just a PostGraphile idiosyncrasy.

### Additional context

Latest `postgraphile` version, 4.12.11.

I'm using PostGraphile in schema-only mode because I freaked out a little when I discovered that PostGraphile's HTTP handler was extending the Node HTTP server to accept websocket connections. Schema-only mode offers a separation of concerns that's very valuable for apps that prefer to fully control the network side.

PostGraphile is absolutely fantastic btw. Thank you :heart: