[5.x] Support pluck() on eloquent query builder

[ryanmitchell]

This PR adds support for pluck() on the base eloquent query builder to bring parity with the stache builder.

This allows you to do queries like:

\Statamic\Facades\Entry::query()->pluck('title');

instead of

\Statamic\Facades\Entry::query()->get()->pluck('title');