Fixes #47

@tuyakhov this is not as pretty as I'd like, but was intended to continue our discussions around how to optionally allow use cases like #47 Happy to have you rip this specific implementation apart, as I have no love for the code 😉 but would like to discuss further thoughts on ways we can get the desired functionality in as I've hit another case where it would be very helpful to have this functionality.

This is a backwards compatible change that allows a consumer to expose data that the serializer can use through the extraFields method:

public function extraFields()
{
    return [
        'car', // documented approach
        'anotherNormalCar' => function($model) { // callback approach that works
            return $model->car;
        },

        // unlocked functionality
        'car' => function($model) {
            return [
                'resource' => $model->car,
                'data' => new ResourceModelIdentifier($model->car_id, 'cars'),
            ];
        },
    ];
}

In using this, exposing callback setups and hashes with names seems fairly fragile. We could potentially expose a different class that internally does something similar. Possibly something akin to this:

public function extraFields()
{
    return [
        'car' => function($model) {
            return new LinkableResourceModel($relationshipMethod, $localForeignKey, $japiType),
        },
    ];
}

Thoughts?