Add ability to create where statements based on related entity

[wolfy-j]

Example:

$user = $user->findOne();
//...
$posts->find()->where('author', $user);

[wolfy-j]

This is important in order to be able repositories more efficient:

public function findByAuthor(User $user): RecordSelector
{
    return $this->findActive(['author' => $user]);
}