SelectRelationActiveQuery relation attribute

[nadar]

The SelectRelationActiveQuery plugin loads the data trough a fix defined where ID= condition, which is wrong and won't work with other primary keys.

Thefore a new attribute relation should be provided. This will also allow the eager loading of relation data in api calls:

public function prepareListQuery()
    {
        return parent::prepareListQuery()->with(['target']);
    }

otherwise this won't have any effect.