Integrate graphiql's support of headers in the explorer

It appears that within [Drupal.behaviors.graphQLRenderExplorer here](https://github.com/drupal-graphql/graphql/blob/8.x-4.x/assets/explorer/src/index.js#L22C1-L30C42), the fetch request does not use the graphiql supported explorer headers.

It looks like the fetch assigns its own headers.

```
// Defines a GraphQL fetcher using the fetch API.
const graphQLFetcher = graphQLParams => fetch(settings.graphqlRequestUrl, {
method: 'post',
credentials: 'same-origin',
body: JSON.stringify(graphQLParams),
headers: {
'Content-Type': 'application/json',
},
}).then(response => response.json());
```

graphiql supports sending custom request headers through its UI as seen in the screen shot below.

Am I looking at this correctly?

It would be sweet to send custom request headers, e.g. Authorization bearer tokens with the request.