Support for "joined" collections

[mnavarrocarter]

It would be nice to add to the main Engine API the possibility of include joined collections.

This could be really easily with an IN clause, although I need to put some care in the implementation.

A possible API:

<?php

$engine->find($ctx, User::class)->of(Group::class, 'userId')->andWhere('id = ?', 1);


$finder = $engine->find($ctx, Group::class)->andWhere('id = ?', 1);
$engine->find($ctx, User::class)->in(Group::class, 'userId', $finder);

[mnavarrocarter]

I'll put this one on the back burner until I build a strong test suite.