Add withCasts to Query Builder

[euoia]

Pretty much the proposal made here:
laravel/ideas#2134

withCasts is available on Eloquent queries, but is not available on the query builder. This would be useful, for example when making JSON columns with DB::raw (e.g. MySQL's JSON_ARRAYAGG).

I can work around it by forcing the query through Eloquent's query builder, but this seems less than ideal (not least because I need to add withTrashed() to prevent query builder adding a WHERE deleted_at IS NULL - even though I am not selected deleted_at in my query).

[vibonacci]

This would be extremely useful for full custom queries that skip Eloquent, yet still want to use casting features of Laravel like created_at -> datetime.