Standard HTTP header to ask for deduplicated data

Instead of recommending to use a query param, why don't we recommend using a custom HTTP header to request deduplicated data?

- `inflate` is a noop if the data wasn't deflated it doesn't matter whether the backend ends up sending deduplicated data or not
- Headers aren't visible in the URL which is neater
- Backend usage is the ~same

I imagine something like this:

```JS
const httpLink = new HttpLink({
credentials: 'include',
uri: '/api',
headers: {
'X-GraphQL-Deduplicate': true
}
});
```

```JS
app.use('/graphql', graphqlExpress((request) => {
return {
formatResponse: (response) => {
if (request.headers['X-GraphQL-Deduplicate'] && response.data && !response.data.__schema) {
return deflate(response.data);
}

return response;
}
};
}));
```

Review the existing recommendation for using a query parameter and the request/response handling shown in the issue. Compare that approach with the proposed X-GraphQL-Deduplicate header, then define the agreed client and server behavior and verify that deduplication occurs only when the header is present.

Written by the indexing model from the issue text.