[8.x] apply where's from union query builder in cursor pagination

[stefanheimann]

adds the ability to use cursor pagination with union queries.

here's an example:

$builder->select('id', 'start_time as created_at')->selectRaw("'video' as type")->from('videos');
$builder->union($this->getBuilder()->select('id', 'created_at')->selectRaw("'news' as type")->from('news'));
$builder->orderBy('created_at');

The specified order for this builder instance is now on unionOrders property rather than orders. But further down the road in src/Illuminate/Database/Query/Builder.php method ensureOrderForCursorPagination, the order is retrieved from orders property, which is null. Within that function, the specified order needs to be retrieved either from orders or unionOrders property, it also needs a filter for orders without direction (type "Raw").

Next issue to address is in method paginateUsingCursor from src/Illuminate/Database/Concerns/BuildsQueries.php. It only adds the conditions from the cursor to the main query, not the union query.

So instead of

(select "id", "start_time" as "created_at", 'video' as type from "videos" where ("start_time" > ?)) union (select "id", "created_at", 'news' as type from "news" where ("start_time" > ?)) order by "created_at" asc limit 17

it produces

(select "id", "start_time" as "created_at", 'video' as type from "videos" where ("start_time" > ?)) union (select "id", "created_at", 'news' as type from "news") order by "created_at" asc limit 17