Custom headers in graphiql.js

**Is your feature request related to a problem? Please describe.**
I'm facing a authentication issue when I use GraphQL playground. My server need a custom header to verify if the request is valid but in graphiql.js have no possible to inject custom header.

**Describe the solution you'd like**
`graphiql.html`
```
window.GRAPHENE_SETTINGS = {
{% if subscription_path %}
subscriptionPath: "{{subscription_path}}",
{% endif %}
graphiqlHeaderEditorEnabled: {{ graphiql_header_editor_enabled|yesno:"true,false" }},
graphiqlShouldPersistHeaders: {{ graphiql_should_persist_headers|yesno:"true,false" }},
graphiqlInputValueDeprecation: {{ graphiql_input_value_deprecation|yesno:"true,false" }},
{% if graphiql_headers %}
graphiqlHeaders: {{ graphiql_headers }}
{% endif %}
};
```

`graphiql.js`
```
var headers = {};
var cookies = ("; " + document.cookie).split("; csrftoken=");
if (cookies.length == 2) {
csrftoken = cookies.pop().split(";").shift();
} else {
csrftoken = document.querySelector("[name=csrfmiddlewaretoken]").value;
}
if (csrftoken) {
headers['X-CSRFToken'] = csrftoken
}
if (GRAPHENE_SETTINGS.graphiqlHeaders) {
var customHeaders = GRAPHENE_SETTINGS.graphiqlHeaders.split(";");
for (const header of customHeaders) {
const [key, value] = header.split(":")
if (key && value) {
headers[key] = value
}
}
}
```
`graphene_django/views.py`
```
self.render_graphiql(
request,
graphiql_headers=self.custom_headers
...
)
```
Usage
```
GraphQLView.as_view(..., custom_headers="header_key_1:header_value_1;header_key_2:header_value_2")
```

**Describe alternatives you've considered**
No alternatives

**Additional context**
N/A