Allow sending back relationship.data hints without includes

[acorncom]

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?

[acorncom]

@tuyakhov any thoughts on this PR? It's been running in production for us without issue for a while now ...

[tuyakhov]

@acorncom I am sorry for taking so long to reply to your PR.
It looks good to me. I was trying to think of other ways to allow this functionality. But couldn't come up with anything better. So I am going to merge this PR and include the changes to the next release.
Would you be able to update the README file as well, so that others know that this functionality exists?
Thank you.

[acorncom]

@tuyakhov apparently I missed your comment here somehow. Still think we want to merge this? If so, I'll update the README and get this ready for a merge

[tuyakhov]

@acorncom Yes, we can definitely merge this one.