Github\ResultPager doesn't support GraphQL

This type of thing could/should work too.

```php
$response = $pager->fetchAll($this->client->graphql(), 'execute', [
'query' => 'query ($login: String!, $perPage: Int!) {
user(login: $login) {
repositories(first: $perPage, orderBy: {field: UPDATED_AT, direction: DESC}) {
nodes {
id
}
pageInfo {
hasNextPage
endCursor
}
}
}
}',
'variables' => [
'login' => 'torvalds',
'perPage' => 10,
],
]);
```

Start at the Github\ResultPager entry point and trace the fetchAll call shown with client->graphql() and execute. Done means the pager supports the supplied GraphQL query and follows its pageInfo and endCursor values to retrieve all repository pages.

Written by the indexing model from the issue text.