X-GraphQL-Event-Stream for being notified that the GraphQL schema has updated

[Server-sent events](https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events) are a perfect mechanism for informing a GraphQL client that the schema has been updated. They don't require the complexity of websockets, and they are uni-directional.

I'm proposing that when introspecting a GraphQL API, if the header `X-GraphQL-Event-Stream` is detected then interested clients should subscribe to the `text/event-stream` at that (relative or absolute) URL and when it receives the `change` event it should automatically re-introspect the GraphQL schema.

Not much code should be required to implement this, just something like:

```js
// refreshSchema is a function you provide which triggers re-introspection

const streamUrl = response.headers["x-graphql-event-stream"];
if (streamUrl) {
const endpointUrl = new URL(endpoint);
const streamUrl = new URL(streamUrl, endpointUrl);
if (endpointUrl.host !== streamUrl.host) {
throw new Error(
`Stream and endpoint hosts don't match - '${streamUrl.host}' !== '${endpointUrl.host}'`
);
}
const eventSource = new EventSource(streamUrl);

eventSource.addEventListener("change", refreshSchema, false);
eventSource.addEventListener("open", () => { /* ... */ }, false);
eventSource.addEventListener("error", () => { /* ... */ }, false);
}
```

(You may want to throttle the refreshSchema method.)

This has been baked into PostGraphile for a while; I opened [an implementation PR](https://github.com/skevy/graphiql-app/pull/120) to @skevy's GraphiQL.app a good while ago; and @imolorhe has [implemented it independently](https://github.com/imolorhe/altair/commit/0b3ccc188597efbe1a57e8f026071743ff6ebffc) in Altair. Samuel wrote more about it here: https://sirmuel.design/a-better-graphql-developer-experience-with-x-graphql-event-stream-1256aef96f24. We'll probably be implementing it in the new version of GraphiQL too, once we have the plugin system in place.

What are your thoughts?