[Feature] Allow primary key loader to lookup by relation

[dkarlovi]

Q	A
Bug report?	no
Feature request?	yes
BC Break report?	no
RFC?	no
Branch?	master

If object_resolver is attached to the property which is a primary identifier, we could run it before the object is looked up. If it matches an object, we load by its ID.

This would allow stuff like

Supplier | Supplier ID
Newegg   |  12345

to be used as a primary identifier, basically using our Supplier object and their (supplier-specific) ID to look up our own object:

TLDR

1. we run the object_resolver which is defined with Name field
2. it finds /orgs/newegg, ID=123
3. we add contion:

$condition[] = '`'.$map->getToColumn().'__id` = ?';
$conditionValues[] = $object->getId();

$condition[] = '`'.$map->getToColumn().'__type` = ?';
$conditionValues[] = 'object';