Simplify ResultPager instantiation

[spawnia]

This simplifies the usage as follows

$manager = app(GitLabManager::class);
-$pager = new ResultPager($manager->connection());
+$pager = $manager->resultPager();

More importantly, it improves discoverability.
Method completion on GitLabManager now provides access to the ResultPager.

[GrahamCampbell]

Thanks for the idea here, though I'm not sure this makes much of a difference since we still need to get the client object anyway to call something like ->issues(). I'll have a think on how to do this in a more slick way.

[spawnia]

I do not get your point. What do you mean by the client object? For example, the call to ->issues() is indeed simplified by this.

-(new ResultPager($gitlab->connection()))
+$gitlab->resultPager()
    ->fetchAll($gitlab->issues())

There might be ways to improve this further, but why not include a small incremental improvement first?