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.

索引モデルが issue の本文から書いたものです。