[12.x] Add Closure-support to $key/$value in Collection pluck() method

[ralphjsmit]

This PR adds closure support to the $key/$value parameters of the pluck() methods. Very often I have that I almost can use pluck(), but I cannot because I need to make a slight modification to e.g. the key or value. I then need to solve this with mapWithKeys(), but that repeats the id and makes it way more verbose if you only just want to apply some formatting to the key and/or value.

After this PR, you can now do the following:

Country::get()
    ->pluck(fn (Country $country) => "{$country->flag} {$country->name}", 'id')

This would result in a collection keyed by id with the concatenated name as value. Eloquent collections are a common use case, but this also works on regular collections and the Arr class.

Thanks!