Controller hooks for modifying relationships should receive the relation value

[lindyhopchris]

As per this issue:
cloudcreativity/laravel-json-api#264

The controller hooks for modifying a relation should receive the relation value. We can use the LazyRelation value here, just like the authorizers.

The LazyRelation class actually caches its value internally for a to-many relation. We should add caching of the to-one relation as well. Then we should keep the instance on the request, so that the same instance is used for both the authorization and the controller hooks: meaning there would only be one database request if the relation is accessed in both the authorization code and the controller hook.

[lindyhopchris]

Actually don't think I want to make any changes at this stage. The second hook (after the relationship is updated) has access to the models that replaced, added-to or removed-from the relation. The main issue with providing them to the before hook is that the records aren't materialized at that point.

I would need a really good use-case to make changes, as the whole point with the current set up is that the JSON:API identifiers are passed down to the repository, which takes care of resolving them and modifying the relationship. It feels like use-case would be solved by using the after hook.