[14.x] feat: support subqueries in whereKey

[calebdw]

Hello!

This add subquery support to whereKey and all related methods!

Thanks!

[taylorotwell]

Hm, when would you need this?

[calebdw]

There's been a couple of cases where we've fallen back to using whereIn('id', ...) because whereKey did not support it

here's an example of the most recent snippet I came across that motivated this PR

            'users' => User::query()
                ->whereIn('id', Accounting::query()
                    ->select('created_by')
                    ->whereNotNull('created_by')
                    ->distinct())
                // ...