[feature] add a conditional separation between showRelated and showRelationship

[Mina-R-Meshriky]

regarding issue laravel-json-api/core#6
if method showRelated exists in the custom authorizer, the two relationship-routes :
/api/resource/{resource_id}/relatedResource
/api/resource/{resource_id}/relationships/relatedResource

will authorize differently

[lindyhopchris]

Thanks for submitting this. However the intent was to add the showRelated method to the authorizer interface.

I've now done that, so closing this PR in favour of 1f12602

Would be useful if you could try out the changes. To do this:

composer require laravel-json-api/laravel:dev-develop --no-update
composer up laravel-json-api/*

[Mina-R-Meshriky]

@lindyhopchris I tested it and it works as expected 👍👍

[lindyhopchris]

Great, thanks for confirming!